Astronomer Vera Rubin discovered that galaxies rotate too fast to be held together by visible matter alone, suggesting the presence of invisible "dark matter." Whether this dark matter consists of heavy particles or some other phenomenon ______ remains a central question in cosmology.
6
readingWhich choice completes the text so that it conforms to the conventions of Standard English?
A
is what
B
and
C
?
D
phenomenon
Correct Answer: D
Choice D is the best answer (with no additional punctuation or words). The clause "Whether this dark matter consists of heavy particles or some other phenomenon" functions as the noun clause subject of the sentence. The main verb is "remains." No punctuation or additional words are needed between the subject and the verb.