21. Helping others is a habit, ________ you can learn even at an early age.
A. it B. that C. what D. one
22. The Hengxin Square is located in ____________ used to be our canteen.
A. what B. where C. which D. the place
23. Mr. Black is very happy because the clothes made in his factory have never been _________.
A. popular B. more popular C. most popular D. the most popular