( )I really don't know _____A.what should I do B.what to do C.what do I

B.what to do
what to do是特殊疑问词加不定式组成的短语,作句子的宾语.又如：
I don't know when to come.
I don't understand why to take part in the party.
Can you tell me how to finish the homework?
A项应改成what I should do,也就是改成陈述语气.
C项应改成what I do,道理同上.
关于陈述语气和疑问语气：
He likes English.这是一个陈述句,是陈述语气.
把它变成一般疑问句就是：Does he like English?
这个句子是陈述语气.它借了一个助动词did提到主语前面.
又如：
He is a teacher.(陈述语气）----Is he a teacher?（疑问语气,系动词is提到主语前面了.）