I suppose they must be trying out new ideas in the lab,__________.A.don`t I B.mustn`t theyI suppose they must be trying out new ideas in the lab,__________.A.don`t I \x05\x05B.mustn`t they C.aren`t they \x05D.needn`t they为什么选D呢?求详解

I suppose they must be trying out new ideas in the lab,__________.A.don`t I B.mustn`t theyI suppose they must be trying out new ideas in the lab,__________.A.don`t I \x05\x05B.mustn`t they C.aren`t they \x05D.needn`t they为什么选D呢?求详解
I suppose they must be trying out new ideas in the lab,__________.A.don`t I B.mustn`t they
I suppose they must be trying out new ideas in the lab,__________.
A.don`t I \x05\x05B.mustn`t they C.aren`t they \x05D.needn`t they
为什么选D呢?求详解

I suppose they must be trying out new ideas in the lab,__________.A.don`t I B.mustn`t theyI suppose they must be trying out new ideas in the lab,__________.A.don`t I \x05\x05B.mustn`t they C.aren`t they \x05D.needn`t they为什么选D呢?求详解
前面是must 表示的是猜测 不是必须 因此后面用needn't 而一般mustn't表示禁止 语气强硬

答案有错误！应该选C。因为must be doing sth.是对现在正在发生的事情的肯定推测。改一下就好理解了：They are trying out new ideas in the lab, aren't they ?

C jim说得对