This old custom has________ since the 18th century, and remains practically unchanged.

natural parks are very important for preservingmany animals, which would ______ run the risk of becoming extinct．

Please don’t talk loudly here. The students _________the teacher in class.

How many students did you see playingin/on the playground________

－＿＿ five yuan enough for a tin of dog food －Of course not.