prep 1-113

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.

(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

E从平行上讲没错。就是有一个疑问，这里warmer than usual怎么补全？感觉应该是usually啊。