是新概念英语第三册上面第一单元的.选项我只写了我觉得比较混淆的.

是新概念英语第三册上面第一单元的.选项我只写了我觉得比较混淆的.
1.People said______the puma.
A.to have seen
B.they had seen
2._____,it immediately ran away.
A.On being observed
B.Having been observed
3.Pumas never attack a human being except___cornered.
A.being
B.when they are
4.The experts were now fully convinced that the animal______a puma.
A.must be
B.could only have been
正确答案我都知道,但是我也不知道我自己错在哪里了.请一定要写出您认为的正确答案和理由噢!
爱BU娃娃
能告诉我第二题选择A更加细致一点的原因跟第四题两个选项的意思（因为你说根据语义来选的）么？还有，第三题我也选的A，
----------------
小精灵
四题那个B。是怎么选的呢？

1.People said ...这里是一个典型的宾语从句,省略了that.
爱BU娃娃所说的said后不能接动词不定式是片面的,在said表示被动含义的时候,后面也是可以接动词不定式的.比如He is said to be generous.据说他很大方.等于It is said that he is generous.
2.On doing something=As soon as sb finished something,是介词On加现在分词短语构成的时间状语,意为在做完某件事情之后（紧接着）,On可以省略.
如：On being observed,it immediately ran away.
等于 Being observed,it immediately ran away.
或者 As soon as it was observed,it immediately ran away.
B选项是时态错误,因为两个动作发生的时间很接近,一般前后用同一时态.
4.must be 表示主观上肯定的推测,意为肯定是,could only have been 表示综合得到的信息,一定是（只可能是）.根据the experts now fully convinced that来看,专家们已经有了足够的证据来断定那只动物一定是（只可能是）美洲狮.
第3题似乎两个都可以,既然答案是B,由于我自己的第一选项也是A,我在这里就不解释了.