I wonder why ________ possible for you to do the work by yoursel

I wonder why ________ possible for you to do the work by yourself.
A、you
B、you are
C、it isnt
D、you were
【正确答案】：C
【题目解析】：本题考查代词的用法。It+ be+形容词+to do是典型句式，it常指代后面的动词不定式。