Read this example of incorrect sentence structure. heavy, black clouds rushing in from the horizon. which revision corrects the sentence? a) heavy, black clouds rushing in from the horizon, get under cover. b) heavy, black clouds rushing in from the horizon. it’s about to rain. c) heavy, black clouds rushing in from the horizon when it’s about to rain. d) heavy, black clouds rushing in from the horizon mean a storm is coming.

It is D because once the heavy dark cloud is rushing in from the horizon it means the storm is coming in

The revision that corrects the sentence is D) Heavy, black clouds rushing in from the horizon mean a storm is coming.

The sentence is incorrect because it is incomplete. "Heavy, black clouds rushing in from the horizon" is only the subject of the sentence; therefore, it must be followed by a verb. "Rushing in from the horizon" is not the predicate of the sentence but a participial phrase working as an adjective modifying the noun "clouds". Therefore, option D) is correct because it is the only one that provides a predicate for the incomplete sentence ("mean a storm is coming").  On the other hand, option A), B) and C) leave the sentence incomplete and add another sentence (in the case of A. and B.) or a subordinate clause instead (introduced by "when" in C.).