I——frustrated when I did not know the answerA may beB used toC used to beD use to be请给我具体的解释

I——frustrated when I did not know the answerA may beB used toC used to beD use to be请给我具体的解释
I——frustrated when I did not know the answer
A may be
B used to
C used to be
D use to be
请给我具体的解释

I——frustrated when I did not know the answerA may beB used toC used to beD use to be请给我具体的解释
C 住过从过

C used to be

c

B，曾经的意思
A：可能
B：曾经
C：习惯于
D：用作于

C.
因为后半句中时态是did，所以空格中应与之相对应，为过去时，故排除A
而B中to 后应加be动词