问题
_______,but it is also a great way to lower the number of greenhouse gases in the air.
A:Not only a fun thing is it to do
B:Not only a fun thing to do it is
C:Not only it is a fun thing to do
D:Not only is it a fun thing to do
B:Not only a fun thing to do it is
C:Not only it is a fun thing to do
D:Not only is it a fun thing to do
正确答案是：
