USADream
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.
(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
A 500/550 level
Thunderstorms form when warm,............WHAT should be your question ?? after that becasue, no matter what is followed, has no sense.
What follows is
eitherNow: in D the construction is not clear and being is always 99% of times is wrong
E has: the right idiom either /or; it also has a parallel structure : either becasue / or because. when either/or (or others such construction quite similar as : neither /nor) is followed by a word, it must be repeated after or. If the word is BEFORE either, after or is not needed to be repeated.
hope it is clear