____ was your last vacation Exciting.

_____ teens are using the Internet for both social and intellectual development, they are also facing significant challenges on how to manage their relationships online.

．It’sbest to plant trees(植树)in_____．

I was surfing the Internet _____of successful rendezvous and docking of Shenzhou-VIII and Tiangong-I.

Listen! I hear some birds ___________ thetree.