On sunny days, dark rooftops absorb solar energy and convert it to unwanted heat, raising the surrounding air ______ a light-
colored covering to an existing dark roof, either by attaching prefabricated reﬂective sheets or spraying on a paint-like
coating, helps combat this effect.
temperature; by adding
temperature, adding
temperature. Adding
temperature by adding
Choice C is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is
used correctly to mark the boundary between the ﬁrst sentence (“On…temperature”) and the second sentence (“Adding…
effect”). The gerund phrase beginning with “adding” is the subject of the second sentence, and the verb phrase “helps
combat this effect” describes what adding a light-colored covering can do. 
Choice A is incorrect because a semicolon can’t be used in this way to join the sentence “On...temperature” and the
supplementary phrases that follow. Doing so leaves the verb phrase “helps combat” without a subject and thus results in a
grammatically unconventional sentence. Choice B 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. This choice results in a confusing and illogical
sentence that suggests that adding a light-colored covering to an existing dark roof raises the temperature of the
surrounding air. Furthermore, it creates ambiguity by leaving the verb phrase “helps combat” without a subject (so it isn’t
clear what helps combat the effect).