应该是 at 表原因
be angry atsth 因…而愤怒:
The members of the group are frustrated and angry at their lack of power.
该集体的成员因为没什么权力而感到沮丧和愤怒.

比较：
be angry with sb生（某人）的气:
I got terribly angry with him.
我对他非常恼火.

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