It is the prevention of disease_____ its successful treatment that has led to the rapid increase of the world’s population.

30．---- Do you think it's a good idea to make friends with your students?---- _____ , I do. I think it's a great idea.

Bob is ________crazy about Beethoven that he knows all his works well.

The national parks are very important for the protection of these wild animals, which would ______ run the risk of becoming extinct.

The young couple couldn’t afford a new house. ____, they wouldn’t have bought an old one.