I can't get though to the general manager's office anyhowThe line is busy.Someone__the phone.A.must useB.usesC.must have been usingD.must it have been是B，为什么？

I can't get though to the general manager's office anyhowThe line is busy.Someone__the phone.A.must useB.usesC.must have been usingD.must it have been是B，为什么？
I can't get though to the general manager's office anyhow
The line is busy.Someone__the phone.
A.must use
B.uses
C.must have been using
D.must it have been
是B，为什么？

I can't get though to the general manager's office anyhowThe line is busy.Someone__the phone.A.must useB.usesC.must have been usingD.must it have been是B，为什么？
这个题目不合理撒
根本就是用排除法来做的嘛,没有考察意义!
你可以看出的,A错 太绝对
C错“已经正在用”?胡扯!
D错 简直乱套!

Someone三单，动词加s

must be using