Could you give me the correct answer and tell me why?Thanks in advance.
1 __,it immediately ran away. A On being observed B Having been observed

2 Pumas never attack a human being except__cornered.
A when they are B they are

3 The experts were now fully convinced that the animal__a puma.
A could only have been B should have been

4 The evidence made the experts__the animal was a puma.
A to think B thinkThanks , the third question's anwer is "could only have been".

Question

Could you give me the correct answer and tell me why?Thanks in advance.
1  __,it immediately ran away.  A On being observed   B Having been observed

2 Pumas never attack a human being except__cornered. 
A when they are   B they are

3 The experts were now fully convinced that the animal__a puma.
A could only have been   B should have been

4 The evidence made the experts__the animal was a puma.
A to think   B thinkThanks , the third question's anwer is "could only have been".

Answer

1 B 
Having "been" observed is past tense and agrees with the past tense verb "ran". 

2 A 
The "when" clause in the present tense indicates habitual action. Pumas usually attack when they "are" cornered. "Are cornered" is present tense passive voice. 

3 B 
"Should have been" is wrong because it indicates "moral obligation". "Could have been" indicates possiblilty. 

4 B 
When using  make + object +infinite the form without "to" is used. 

You have to memorize which form is normally used with the main verb in the sentence. 

For instance, if the main verb in this sentence were "to cause", then you would use the "to"form. 

Example: 
The evidence caused the experts "to think" the animal was a puma. 

Денис

Answer

1,A.  On being observed    译为 ：   一……就
2.A   ？
3.B  虚拟语气 have done是对现在（now）的虚拟，因为用在过去时中，就是对过去的虚拟-->should have done。
4.B  make sb do 固定搭配