all the seats were full, he had no choice but to stand up.A. ForB. AsC. ThoughD. Unless

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A. For
(答案→)B. As
C. Though
D. Unless
解析：考查连词的用法。For 因为，只能放在句中不能放在句首，As 因为，Though 虽然，Unless 除非，句意：因为所有的座位都满了，他别无选择除了站着。所以选B。