The intense pressure found in the deep ocean can affect the structure of proteins in ﬁsh’s cells, distorting the proteins’
shape. The chemical trimethylamine N-oxide (TMAO) counters this effect, ensuring that proteins retain their original ______ is
found in high concentrations in the cells of the deepest-dwelling ﬁsh.
conﬁgurations. TMAO
conﬁgurations TMAO
conﬁgurations, TMAO
conﬁgurations and TMAO
Choice A is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period
after “conﬁgurations” is used correctly to mark the boundary between one sentence (“The intense…conﬁgurations”) and
another (“TMAO…ﬁsh”). The supplementary phrase (“ensuring…conﬁgurations”) modiﬁes the main clause of the ﬁrst
sentence (“The chemical…effect”), and “TMAO” is the subject of the second sentence.
Choice B is incorrect because it results in a run-on sentence. The sentences  (“The intense…conﬁgurations” and “TMAO…
ﬁsh”) are fused without punctuation and/or a conjunction. Choice C is incorrect because it results in a comma splice. A
comma can’t be used in this way to mark the boundary between sentences. Choice D is incorrect. Without a comma
preceding it, the conjunction “and” can’t be used in this way to join sentences.