NEW DELHI: The Election Commission today threw an "open challenge" to people to hack its EVMs , a move which comes after opposition parties urged it to revert to the paper ballot system raising doubts over infallibility of the machines."From first week of May, experts, scientists, technocrats can come for a week or 10 days and try to hack the machines," an official source said.They said the challenge will be open for a week or 10 days and will have various levels.The Commission had announced a similar challenge in 2009 and it claimed no one could hack its electronic voting machines (EVMs).