1 .During the flood of 1927,the Red Cross,_____out of emergency headquarters in Mississippi,set up temporary shelters for the homeless. A operates B has operated C.is operating D.operating
这一题应该用什么时态?
2.Nearly all trees contain a mix of polymers that can burn like petroleum _____ properly extracted. A.after B.if C.when it D.is
这一题当时答案是选对了,选B,但C有何不可?疑惑
3.Certain zoologists regard cows and ravens are the most intelligent of birds. A.certain B.are C.intelligent D.birds
Good pottery clay must be free from all small stones and other hard materials that would make the potting process difficult.Most North American artisanpotters now purchase commercially processed clay,-----------