Which choice completes the text so that it conforms to the conventions of Standard English? The fine, powdery substance that covers the Moon's surface is called regolith. Because regolith is both readily available and high in oxygen ______blank scientists have wondered whether it could be used as a potential source of oxygen for future lunar settlements.

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.