4. Is swimming under water very difficult? - No, it's just a matter _____ able to control your breathing.
A. to be B. of being C. that you are D. if you had
"B. of being" is correct. However, how can I explain that "C. that you are" is not correct? Many thanks.
I don't think there any other explanation than the fact that a matter of is a fixed phrase, even without able: It's a matter of time.

thanks for your kindness