问题补充：
We all know that we our carbon footprint, but I suspect that many people wouldn 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
答案：
A. must have reduced
(答案→)B. should have reduced
C. may have reduced
D. could have reduced
解析：B 考察情态动词。句义：我们都知道本应该减少碳，但是我怀疑很少有人能够项Joan Pick一样。Must have done表示对过去情况的肯定推测。过去一定…；should have done本应该做某事，实际上却未做。May have done或许…；表示没有太大的把握，根据句义说明B正确。
