15. Which sentence CAN'T be used with 'must'? a. I... train a lot to win the game. It's important for me.

b. The rules say that you... turn on the light when cycling at night.

c. Athletes... wear a specific outfit for gymnastics, but if they want, they can wear a gymnastics suit.

The sentence that can't be used with 'must' is:

c. Athletes must wear a specific outfit for gymnastics, but if they want, they can wear a gymnastics suit.

In this sentence, the use of "must" implies a requirement or obligation, but the phrase "if they want" suggests that wearing a gymnastics suit is optional. The use of "must" contradicts the idea of optional choice.