21. The singer was admitted in 2013 and did not get a diploma from the university when he graduated in 2018, as he did not meet the ________.

A. promise         B. challenges         C. requirements         D. request

22. It was some time before the door opened ________ his ring.

A. on account of    B. in view of          C. with regard to       D. in response to

23. Foreign leaders were wondering how it was that the Chinese government could ________ the spread of the virus so quickly and effectively.

A. defend         B. accelerate           C. contain             D. assure