Among the 15-year-old children, some ____ studying in university while others are still learning to read and write.

The pet dog in your hand is very nice. Isit –Yes, but I’ll give it to my friend, Lucy,as birthday present.

How do you like Guilin?— It is the most beautiful place ______ I have ever been to.

When he walks______my desk，he knocked my books and pens _____the floor．

Wind power is an ancient source of energy ____ we may return in the near future.[ ]