Todd Graham is the director of debate at Southern Illinois University. His debate teams have won five national championships and he has been recognized three times as the national debate coach of the year. Follow him on Twitter . The views expressed in this commentary belong solely to the author. View more opinion at CNN.

(CNN) Wednesday's debate winners weren't the big names, in my view, but a couple of lower-tier candidates. Here are my grades for the candidates:

B

Andrew Yang

Yang is comfortable on stage and it shows. I'm a sucker for a good opening and closing and Yang's were the best. "The opposite of Donald Trump is an Asian man who likes math," was a catchy line from the opening, and making his lack of a necktie a focus in his closing was equally engaging. His answers were crisp and he defended a single theme on almost every topic, such as his universal basic income plan: every adult would get $1,000 a month. He continually came back around to how this would solve problems, from equal pay for women and minorities to criminal justice inequities and even climate change.

Hopefully, we'll get to see him in the September debate round and he can expand his repertoire of answers.

B

Michael Bennet

Bennet is an odd one to figure out. Just when you think he's too boring or plain, he shifts gears and throws out some quality answers. His positions were well defended on the high costs of Medicare for all ( 30+ trillion dollars ) and why its defenders need to level with the American people on the issue.

Bennet was also the best debater on stage on civil rights, the need for a fairer education system, and on how to address criminal justice reform. I know, it shocked me too.

B-

Kirsten Gillibrand

Gillibrand has improved her tone and her answers from the last time she was on the debate stage. She hit themes that resonate with many when she reminded us that she helped pass a bill to permanently fund the 9/11 first responders' health care needs. And her reflections on being a white woman of privilege in our society today were terrifically contextualized when she spoke about how she could communicate this privilege to suburban women who voted for Trump. Gillibrand also had the best line of the night when she said that the first thing she'd do as president would be to Clorox the Oval Office.

Unfortunately, she lost steam when she chose to attack Biden in what appeared to be a disingenuous criticism of Biden's general attitude toward working women.

B-

Cory Booker

He won his skirmishes with Biden, especially when he argued that the former vice president can't have it both ways -- defending the actions of the Obama administration when successful, but shirking his own responsibility when Obama's actions are criticized. Booker also made a point about voter suppression being a threat to our elections and democracy. However, he slowed a bit when defending his actions to fix the policing system as the mayor of Newark.

Booker had a decent debate. It just didn't showcase why he's the best candidate for president -- and that's the focus he needs in future debates.

C+

Kamala Harris

There were no catchy moments for Harris in the debate. She got off to a slow start and didn't seem to have the same tone or spirit that she had in the previous debate. While she went back to criticizing Biden as an offensive tactic, it was less effective once he began to criticize her policies in return. Plus, this was the first debate where Harris was put on the defensive -- and not just from Biden, but from Tulsi Gabbard and even the moderators.

Interestingly, Harris's main problem was the opposite of Biden's. She often spoke too long. When your time is up but you're still building to your main point, you've lost the entire persuasive appeal of your argument.

C

Three candidates receive this grade for the same reason. They got lost in the shuffle. While the following candidates didn't make any major mistakes, they also rarely shone.

Julián Castro

He was likeable and his answers were decent.

Jay Inslee

He stuck with his main theme -- the environment and climate change.

Tulsi Gabbard

Gabbard performed best when she was criticizing Harris, but this wasn't enough to pull her out of the crowd.

C-

Joe Biden

What saved his grade was, in fact, other candidates on stage. Tulsi Gabbard had an effective critique of Kamala Harris's past actions as a prosecutor while Michael Bennet persuasively argued that we need to be discussing the important issues of today, not from 50 years ago. These moments in the debate brought some relief to the former vice president, and from there he was able to hold his own. Indeed, his best answer of the night was in response to de Blasio asking whether the former vice president would oppose a new NAFTA. Biden simply said, "yes."

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Biden proved on stage that he is not the same debater he once was. And it's not because he gets flummoxed easily, nor is it because he stumbles or makes the occasional gaffe (at one point, calling Cory Booker the president).

And while Biden was targeted excessively, his main problem was not his opponents. It was the timer.

I teach my students that the timer is your enemy. Knowing this, Biden needs to simplify his answers, have a main focus, and summarize that focus instead of trailing off mid-sentence when time is up.

D

Bill de Blasio

De Blasio gave off an annoying and sometimes pushy vibe in the debate. I teach my novice debaters a basic point: A question is not an argument. Yes, de Blasio hounded Biden for answers, and it put Biden on his heels. But by confusing questions for arguments, de Blasio presented no reason to vote for him. Plus, his wild shouting about the need to talk about Iran only compounded my negative impression.