the statute is too vague too broad Forum

[unclej]

on mbe questions i often come across this scenario:

C) unconstitutional because blah blah
D) unconstitutional because the statute is too vague too broad

Sometimes I am sure that C is correct, until I see D. in every scenario, if you think about it hard enough, you could argue a statute is too vague or overbroad.

what is your strategy for picking C v D? how do you know when D is correct?