One of the good things about the use of noise-killing system is _________ it saves the need for a silencer.?

A.whether　B.that　C.how　D.why?

One of the good things about the use of noise-killing system is _________ it saves the need for a silencer.

1. A.
   whether
2. B.
   that
3. C.
   how
4. D.
   why