根据句意,用所给动词的适当形式填空。
The teacher told us that the earth       (travel) around the sun.
Trees and grass play an important part in _______ (help) improve our environment．
Many trees___________(plant) in my hometown every year.
CCTV is starting a project _________( call ) “Looking for the most beautiful country teachers.”
We are talking about where ___________(spend ) our coming winter holiday.