21. Teaching students is_______, but the teachers_______.
A. a hard work; hard work B. hard work; hard work
C. a hard work; work hard D. hard work; work hard
22. It_______ five years since he_______ Beijing.
A. was; leaves B. have been; left
C. is going to be; leaves D. is; left
23. -What do you think of summer in Nanjing?
-Well, it's sometimes_______ hot and sometimes there is_______ ram.