52.
Your Answer is
Correct Answer is F
Explanation
There is already a complete sentence before the underlined part, and there is no conjunction in this sentence, so there cannot be a complete sentence after it, G/H error.
Gather as the meaning of [people gathering] is intransitive, so item J is wrong.
F item is an independent nominative test point, that is, a guard rail the only thing separating them from the crater's edge is an accompanying adverbial of the main clause, so it is correct.