?我不确定

选A语法也没错误，主要是根据I'm not sure（我不确定），来确定选D，说明这句话表示的是我的一种猜测，所以选D 3Need I hand in the homework right now?

?我还不确定

I'm not sure yet.|我还不确定 With the super-hearing, does he hear each sound by itself or everything at once?

?我也不肯定

... 那是不对的 That's not right. 瞎说 Rubbish. 我也不肯定 I'm not sure ...