Why do you like playing badminton?

Question

1. Because I want to be healthy.
2. I want to stay healthy.
3. To remain healthy.
(Are all the answers grammatical?)

4. I want to be healthy.
5. I want to stay healthy.
6. I want to remain healthy.
7. I want to keep healthy.
(Which expressions are correct and commonly used?)

Answer 1

All the answers are fine, but I have never heard anyone use the phrasing in 7.

PS -- Don't people play badminton for fun?
=)