If you answered D, and you also gave the correct answers to several much more complicated questions, you are on your way to a job in an increasingly tough and essential field: managing the North American power grid.

“The bar is being raised,” said Lourdes Estrada-Salinero, the director of operations compliance and control at the California Independent System Operator, one of the more than 100 “balancing authorities” that are responsible for coordinating supply with demand in some portion of the North American grid. About 40 percent of all the energy used in the United States — all the oil, gas and coal, uranium, wind and falling water — is turned into electricity before it is consumed, and that fraction seems destined to rise, as more air-conditioners, electric cars and yet-to-be invented hand-held gizmos are added to customers’ inventory. But the tolerance for failure is getting lower, and the power mix is getting more complicated. States, led by California, are demanding that an increasing fraction of the electricity come from sources that can turn themselves on or off with very little warning, as the weather changes and the wind and sunlight vary.

Utility companies used to hire and train their own people to operate their systems. As the grid became more interconnected, some utilities organized themselves into power pools, with one control room handling the supply for multiple utilities. The earliest was the entity now known as PJM, which used to stand for Pennsylvania-Jersey-Maryland but now stretches into all or part of Delaware, Virginia, West Virginia, Ohio, Illinois and scattered parts of other states. New York had a power pool and so did the six New England states.

The job got exponentially more complicated when the federal government pressed the pools to convert into power markets, where the utilities would sell off their generating stations and third parties would be allowed to build generators. The hour-by-hour decisions about who would generate to serve what load were made mostly by an auction process and turned the pools into “independent system operators.”

The human operator’s job, though, was mostly still unregulated until August 2003, when a series of errors in a control room in Carmel, Ind., at the Midwest Independent System Operator, created the biggest blackout in history. (For future reference, when disabling vital computer systems to install upgrades, kindly do not neglect to warn the system operators and do not leave the systems shut off when you go to lunch.) The lights went out as far away as New York.