1.I didn’t hear the phone.I ___ asleep.
问题描述：
1.I didn’t hear the phone.I ___ asleep.
A.must be
B.must have been
答
B must be 是对现在肯定的猜测“一定”
而must have done是对过去发生的事做肯定的猜测.
我（当时）没有听到电话,我（当时）一定睡着了.