Question

________, but it is also a great way to lower the number of greenhouse gases in the air．

1. A.
   Not only a fun thing is it to do
2. B.
   Not only a fun thing to do it is
3. C.
   Not only it is a fun thing to do
4. D.
   Not only is it a fun thing to do

Answer 1

D
not only放句首句子要部分倒装。