pumas never attack a human being expect _____cornered.A.they areB.beingC.that they D.when they are

楼主打错了吧,应该是except.做连词,意思是“除非, 除了 ... 之外, 若不是”.
整个句子意思是：美洲狮从来不攻击人类,除非被逼入困境.
except的相关句式/短语如下：（没有接动名词的）
1.except+名词
　The office is open every day except Sundays.
　 You can have any of the cakes except this one.
2.except for
　 Everyone went except for Scott and Dan.
　 She felt fine except for being a little tired.
3.except (that)
　 Clarissa could think of nothing to say except that she was so sorry.
4.except in/by/to etc
　 Staff are not permitted to make personal phone calls except in an emergency.
5.except when/where/if
　 Benson kept the studio locked except when he was working there.
6.except do sth
　 She had nothing to do except spend money.
7.except to do sth
　 He wouldn't talk about work, except to say that he was busy.
8.used to give the reason why something was not done or did not happen
except (that)
　 Liz would have run, except that she didn't want to appear to be in a hurry.
9.spoken used to mention a fact that makes what you have just said seem less true
except (that)
　 I have earrings just like those, except they're blue
　 A date book would make a great gift, except that a lot of people already have one.