Green roofs can reduce storm-water runoff while providing habitat for insects. The number of urban rooftops that host gardens _____ native flowering plants remains relatively small.
Which choice completes the text so that it conforms to the conventions of Standard English?
When a sentence contains several nouns, locate the subject associated with each verb instead of automatically matching a verb to the closest noun. Temporarily remove prepositional phrases such as “of urban rooftops” to expose the main subject, then identify the noun to which each relative pronoun refers.
Hints
Separate the clauses
Determine which noun is the subject of the verb describing an increase and which noun the relative pronoun “that” refers to.
Ignore intervening nouns
For the first verb, focus on the main noun before the phrase beginning with “of,” rather than on the plural nouns inside that phrase.
Check the relative clause
Ask what can logically contain native flowering plants: the proportion or the gardens.
Step-by-step Explanation
Identify the first verb's subject
The subject of the first clause is “number,” not the nearby plural nouns “rooftops” or “gardens.” Because “number” is singular, it takes the singular verb “has risen.”
Find the antecedent of “that”
In “those gardens that contain native flowering plants,” the relative pronoun “that” refers to “gardens.” The plural noun “gardens” requires the plural verb “contain.”
Combine the two agreement decisions
The sentence therefore needs “has risen” for “number” and “contain” for “gardens,” making choice D correct.