两个选项都错.
A.错在无论so .that或so.句型,后面是个完整的句子,do是及物动词,后不能少宾语it.B.错与A相反,too.to句型,do已有宾语,就是前面的exercise.故不能再跟宾语it.
This exercise is so difficult that few of us can do it或This exercise is difficult,so few of us can do it.或 This exercise is too difficult for any one of us to do才是正确表达.