A group of ecologists led by Axel Mithöfer at the Max Planck Institute for Chemical Ecology in Germany examined the
defensive responses of two varieties of the sweet potato ______ TN57, which is known for its insect resistance, and TN66,
which is much more susceptible to pests.
plant.
plant;
plant
plant:
Choice D is the best answer. The convention being tested is punctuation use between a main clause and a supplementary
phrase. In this choice, a colon is correctly used to mark the boundary between the main clause (“A group…plant”) and the
supplementary element (“TN57…pests”) and to introduce the following elaboration on the speciﬁc varieties of sweet potato
plants that were examined.
Choice A is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “TN57.”  Choice B is
incorrect because a semicolon can’t be used in this way to join the main clause (“A group…plant”) and the supplementary
element (“TN57…pests”). A semicolon is conventionally used to join two main clauses, whereas a colon is conventionally
used to introduce an element that explains or ampliﬁes the information in the preceding clause. Choice C is incorrect
because it fails to mark the boundary between the main clause (“A group...plant”) and the supplementary element
(“TN57...pests”) with appropriate punctuation.