The ﬁne, powdery substance that covers the Moon’s surface is called regolith. Because regolith is both readily available and
high in oxygen ______ scientists have wondered whether it could be used as a potential source of oxygen for future lunar
settlements.
content and
content,
content
content, and
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.