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题库 / Prep2007E1-SC-113
Thunderclouds form when warm, moist air rises into cooler air above, because either the ground is warmer than usually or because the interaction of two air masses, one warm and one cold, forces warm air to rise.
because either the ground is warmer than usually or because
because either the ground is warmer than usual or because of
because of either the ground being warmer than usual or because
either because of the ground's being warmer than usually or because of
either because the ground is warmer than usual or because
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either or排除A,B,C
or后面跟的是原因状语从句，either后面也应该跟从句→句子要读完，不要光光只看到the interaction就判断because of。
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