问题补充：
,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
答案：
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
(答案→)D. Not only is it a fun thing to do
解析：not only放句首句子要部分倒装。
