Plastic bags are provided for our convenience but they have also ___ the pollution of the environment.[ ]

Travelling in a big city isn’t that difficultRaise your hand, and a taxi will appear_____

One way to ______ species under threat of extinction is to remove them to zoos and parks and feed them there.

The Great Wall is a place ______ almost all tourists would like to visit when they come to Beijing．

_____, I think, and the problem can be settled.