21．Jumping out of ______ airplane at ten thousand feet is quite _____ exciting experience.
A. an; an B. /; the C. /; an D. the; the
22．The boy pretended _when his mother entered.
A. reading B. to read C. to be reading D. being read
23． _ ，and you’ll find the city more beautiful._from the top of the building
A. See B. To see C. Seeing D. Seen
