21£®The amazing caves of Zhoukoudian are      extremely precious part of our cultural heritage and it is of      vital importance that we do something to protect them.

A.a;/   B.the;an    C.an;/    D.an;a

22.Other things being equal, a man who expresses himself effectively is sure to succeed more rapidly than a man whose      of English is poor.

A.writing    B.program    C.command    D.impression