一楼是正确的选B
这是个 I'm sure 引导的宾语从句,后面通常接一个省略句 they were
I'm sure 也常用作表示询问的回答,IS this the biggest apple
yes,I'm sure
a I think 其后接完整句子 eg I think they were working er而不能用I think they were
其他的两个楼主能判断的