The difference in meaning between could take / could be taking / could have taken

A task from the Thinking Skills exam:

Of the 24 students in a class, 11 take German and 16 take Chemistry. What is the smallest number that could take Chemistry but not German?

Now, here are 5 versions of the task question:

1) What is the smallest number that could take Chemistry but not German?
2) What is the smallest number that could be taking Chemistry but not German?
3) What is the smallest number that could take Chemistry (in the future?) but not German?
4) What is the smallest number that could take Chemistry (though they didn't do it?) but not German?
5) What is the smallest number that could have taken Chemistry but not German?

How would you answer each of these?

How has the assumed implication in the parenthesis influenced your response?