Experts have been warning of the health risks caused by passive smoking33.Experts have been warning ____ of the health risks caused by passive smokingA.at a time B.at one time C.for some time D.for the time为什么选择C?

have been to do什么意思?他和have been doing,have been done,have done有什么区别?例句：Chinese authorities say Russian experts have been on hand to support the entire mission. some experts have gone further是什么意思 高中英语小题bob is the only one of those foreigh experts who --------in china now A worked B is working C are working D have been working 新概念上的一道选择题The experts were now fully convinced that the animal_______a puma.A.must be B.should have been C.can only be D.could only have been可能是多项选择,是A和D么 the experts were now fully convinced that the animal ___a puma1 must be 2 should have been 3 can only be 4 could only have been请问选择哪一个,及分别说出每个为什么对与错课文里面有一句话The experts were fully convinced that The experts were fully convinced that the animal ________a puma.A:must be B:should have been C:can only be D:could only have been我自己感觉应该是选择A课文里面有一句话The experts were fully convinced that the animal was a puma.所以 Experts have been warning of the health risks caused by passive smoking33.Experts have been warning ____ of the health risks caused by passive smokingA.at a time B.at one time C.for some time D.for the time为什么选择C? I have been asked to a party,but I have got nothing to wear.这个句子中的have been asked为什么不能换成wa asked?这一类的时态题该怎么作? Wa all have our crosses to have been have been have been have been (3)Computer experts have been gripped by fear of a dangerous illness.It is highly contagious,difficult to diagnose and in many cases impossible to cure.It is not computer experts or programming staff who are likely to be affected,but rather the elect The costs of higher education,as well as poor preparation for college,___many minority students,experts say.A are holding back B holds back C have been held back D has held back麻烦说明一下原因. 2.Mr.Herpin is one of the foreign experts who ______ in China.A.works B.is working C.are working D.has been working3 .Tom is the only one of the students who ______ to Shanghai.A.have gone B.have been C.has been D.had gone 1 （）,it immediately ran away.a.Observing her b.On being oberved c.Having been oberved d.On her being oberverd2 The experts were now fully convinced that the animal ( ) a puma.a.must be b.should have been c.can only be d.could only have been Mr.Herpin is one of the foreign experts who____in China.A:works B:is working C:are working D:has been workingTom is the only one of the students who____to Shanghai.A:have gone B:have been C:has been D:had gone这两道题的先行词分别时什么,