pronouns.

The correct answer is B: She is very nervous before some exams, but not when she feels she's done enough revision. Logic should tell you this, because 'every...' and 'but not when...' are contradictory.

Every is only used before singular nouns. In English, a period or interval of time although expressed using plural time units e.g. five minutes, six hours, two days, five years, is singular: I check my email every five minutes, a bus leaves every six hours, I drink water every two hours, there is an election every five years.

Thanks a lot!