When you don’t know the answer
It’s not that this student didn’t want to answer this chemistry question, but the government wouldn’t allow him to. There’s a huge “TOP SECRET” cover on top of the answer, which would indicate that whatever is under there is classified information. This sort of thing happens all the time when exam season is upon us.

We gotta admit that this is an incredibly creative way to hide the fact that you don’t know the answer to a question. Creative, but you’re not going to get full marks for your answer. Just so you know, TOP SECRET is universally understood as “I don’t know.” Even this teacher from Indonesia could understand that!