Possible Duplicate:
“Which” vs. “what” — what's the difference and when should you use one or the other?
I know that, in the context of radio buttons (the options are limited and they choose one), I should use which. For example, Which is your favorite color?
( ) blue ( ) green ( ) red
I also suspect that for an unlimited text box, the correct option is what. For example, What is your favorite color? ________________
My instinct tells me that the same is true for checkboxes. For example, What colors do you prefer?
[ ] blue [ ] green [ ] red
When they can select multiple answers, I should use What colors do you prefer?, not Which colors do you prefer?, right?