问题
It’s clever _____ you to help me work out this difficult maths problem. Thanks.— You are welcome. Now it’s easier ______ you to understand it.
A:is used; not to pollute
B:has used; is not polluted
C:is being used; do not pollute
D:has been used; to pollute;
B:has used; is not polluted
C:is being used; do not pollute
D:has been used; to pollute;
正确答案是：
