新概念NCE3 第一课教材中的一道练习题 关于except的用法：

新概念NCE3 第一课教材中的一道练习题 关于except的用法：
Pumas never attack a human being except( ）cornered.
A.they are
B.being
C.that they are
D.when they are
不解.但可以肯定的是,这里的Except表示的是“除非”.
因为查了朗文,当表示“除非”的时候,有如下用法：
1 used to introduce the only person,thing,action,fact,or situation about which a statement is not true :
The office is open every day except Sundays.
You can have any of the cakes except this one.
except for ：
Everyone went except for Scott and Dan.
She felt fine except for being a little tired.
except (that) ：
Clarissa could think of nothing to say except that she was so sorry.
except in/by/to etc：
Staff are not permitted to make personal phone calls except in an emergency.
except when/where/if：
Benson kept the studio locked except when he was working there.
except do something：
She had nothing to do except spend money.
except to do something：
He wouldn’t talk about work,except to say that he was busy.
GRAMMAR
Do not say 'except of something' or 'except from something'.
Except is not used by itself to introduce a clause.Use unless or except when/where/if :
We won’t go unless you really want to (NOT except you really want to).
I cycle to work,except when it rains (NOT except it rains).
按照朗文给出的用法：
除了b being肯定不能选之外,其他都是可选的.为什么答案是D when they are?
问题在于什么情况下用excep (that)?什么时候用except when?
像这题如果选a they are,c that they are 也说得通啊?