第二部分: 单项选择（共15小题；每小题1分，满分15分）
21. helicopter can take off and land straight up or down, and can also stay
still in air.
A. The; an B. A; the C. A; 不填 D. The; 不填
22. is known to us all is the old scientist, for whom life was hard in the
past, still works very hard in his eighties.
A. What; what B. What; that C. That; what D. As; that
