pumas never attack a human being except ___cornered.

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pumas never attack a human being except ___cornered.
1 they are 2 being 3 that they are 4 when they are
请问选择哪一个,及分别说出每个为什么对与错

答案是 4 .
EXCEPT在这里是介词,後面不好直接加句子,所以要用that / when 链接,在加句子.
1和2就被排除了.
3 被否定,是因为 except that + 句子------强调这样的概念：------强调事实的陈述,不强调事情发生的时间.
He doesn't have A ,give B or take C except that he enjoys D.
4 强调的是“时间",对时间注重：
pumas never attack a human being except WHEN they are cornered.
== pumas don't attack a human being WHEN they are NOT cornered.
== PUMAS WILL ATTACK A HUMAN BEING when IT IS CORNERED
你也可以这样看：
pumas won't attack a human being IF they are NOT cornered.
==pumas won't attack a human being EXCEPT IF they are cornered.

pumas never attack a human being except ___cornered. Pumas never attack a human being except( )cornered? Pumas never attack a human being except( )cornered?选项：A they Pumas never attack a human being except when the are cornere Pumas never attack a human being except( )cornered?这新概念的老问题哈 pumas never attack a human being expect _____cornered.A.they Pumas never attack a human being except_____conered. A.that Pumas never attack ahuman being except being cornered.这句话哪里有 only when it is hungry will a lion attack a human being. 英语的虚拟语态新概念第三册第一课就有一个句型:a puma will not attack a human being a human being a human kind human,human being