23. We had been away for twenty years. You______ imagine how delighted we were when meeting
A, must B. can C. will D. might
答案给的B,must也表推测，选must可以吗？在这题怎么区别？

must不是推测的意思而must be才是推测的意思。你看你自己出的例句就清楚了。
It must be raining.
It must be true.
这是因为must be 作为推测是，实际的句法是：
it must be the case that....
即:
it must be the case that it is raining
it must be the case that it is true.
更多例子：
he must be lying=it must be the case that he is lying. 他一定是在撒谎。
he must be a southerner=it must be the case that he is a southerner. 他肯定是个南方人。
must+除了be以外的动词没有推测的意思。所以这道题要选can而不能选must。