单选
21. Students should be encouraged to use ____Internet as ____resource.
A. 不填; a B. 不填; the C. the; the D. the; a
22. He found it increasingly difficult, _____his eyesight was beginning to fail.
A. though B. for C. but D. so
23. _______was most important to her, she told me, was her family.
A. It B. This C. What D. As
