21. There are over 58, 000 rocky objects in _______ space, about 900 of which could fall down onto _______ earth.
A. the;the B. 不填;the C. the;不填 D. a; the
22. (2012·郑州高二检测)The doctor will not permit him _______ until his foot is better.
A. walk B. to walk C. walking D. walked
