为什么选B，A应该也讲得通吧？

Question

We all know that we ________ our carbon footprint, but I suspect that many people wouldn't be prepared to go as far as Joan Pick, a retired scientist living in south London.
A．must have reduced B．should have reduced
C．may have reduced D．could have reduced

Answer 1

A的意思是一定已经做过，但有下文“I suspect that many people wouldn't be prepared ……'我怀疑很多人并没有准备像Joan做的这么好，这与must have done表明对过去的肯定推测是矛盾的；而B的意思是本应该做但没做，这个更与下文相符。

Answer 2

should have done是“本应该……”，是虚拟语气的常见用法；意思是本应该发生的事没有发生；而must have done是绝对肯定的语气，表示一定会发生的事情，从句中意思可知，应该是“本应该……”所以选B不选A

Answer 3

A是表示肯定猜测。B是本应该但没有。

Answer 4

A选项是中国式的英语，B选项是根据这个语句的时态选择。另外一点就是must这个是必须一定的口气，而should更适合这个语气词