—What do you think of the 3-D printer?—It’s great although it still needs further       .

On Teachers’ Day, I sent a ________to my English teacher, and said “Happy Teachers’ Day” on it.  

____ mothers can’t go to the meeting, because they have gone to Beijing.

--- Did the book give the ___________ you needed?--- Yes. But ____________ it, I had to read the whole book.

The air pollution is very serious,we should take___________to stop it.