80、 hunderclouds 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.
　　(A) because either the ground is warmer than usually or because
　　(B) because either the ground is warmer than usual or because of
　　(C) because of either the ground being warmer than usual or because
　　(D) either because of the ground's being warmer than usually or because of
　　(E) either because the ground is warmer than usual or because
　　原文意思：
　　云的形成或者是因为。 。 。或者是因为。 。 。 。
　　题目分析：
　　原文有一个并列，就是两个原因的并列，A 选项 either 放在 because 后面两个原因就不并列了， 干掉。 B 前面是 because 后面是 because of， 不并列， 干掉。 C 前面是 because of 后面是 because 不并列， 干掉。 Deither 后面跟 because of ， 后面 or 后面跟 because of，写的还比较并列，留着。E either 后面跟 because，or 后面也跟 because，比较并列，也留着。D 和 E 进行比较，D 的 being 表达的是“正在进行”的意思，而这句话表达的意思不是地表正在比平时温暖的意思，语义不对，选 E。