The fine, powdery substance that covers the Moon’s surface is called regolith. Because regolith is both

Choice B is the best answer. The convention being tested is punctuation between a subordinate clause and a main clause. This choice correctly uses a comma to mark the boundary between the subordinate clause (“Because...content”) and the main clause (“scientists...settlements”).

Choice A is incorrect. Joining the subordinate clause (“Because...content”) and the clause that follows (“scientists...settlements”) with the conjunction “and” results in an ungrammatical sentence that lacks a main clause. Choice C is incorrect because it fails to mark the boundary between the subordinate clause and the main clause with appropriate punctuation. Choice D is incorrect. Joining the subordinate clause (“Because...content”) and the clause that follows (“scientists...settlements”) with a comma and the conjunction “and” results in an ungrammatical sentence that lacks a main clause.