An agreement question with two changing verbs tests whether you can match each verb to its own subject. Sort the forms by number: singular verbs go with one thing, and plural verbs go with more than one. Then find each subject, setting aside phrases that only describe it. In a describing clause with that, ask which noun the clause describes, and check with it or they. A nearby noun may belong to a different clause.
Hints
- Hint 1
In the second sentence, ask what rises. Set aside the describing words beginning with of when you answer.
- Hint 2
The sentence ends with “remains relatively small.” For the verb beside “native flowering plants,” ask which noun the describing clause is about.
Step-by-step
Match each verb to its own subject
Step 1Sort the two verbs
Compare the verbs that change: one follows gardens, and another comes before native flowering plants. We need a separate subject-verb agreement check for each, which means matching each verb to its own subject. Which forms go with one thing? The singular forms are has risen and contains; plural have risen and contain go with more than one.
- Step 2
Find what rises
Ask what has gone up. In The number of urban rooftops that host gardens, the phrase beginning with of tells us what is being counted, so let's set it aside. That leaves one number as the subject. The plural verb host belongs to the describing clause about rooftops, so it doesn't say how the number has changed.
- Step 3
Find what holds the plants
Which noun contains native flowering plants? When that is the subject of a describing clause, its verb matches the noun the clause describes. In the proportion of those gardens that, the clause describes gardens, because gardens can hold plants, while a proportion tells us what share of them do. The proportion is what remains relatively small, and that verb is already present. So the second missing verb matches the plural gardens, even though proportion is singular.
- Step 4
Use the pronoun check
Try it and they in place of the two subjects. We get it has risen for The number and they contain for the gardens holding native flowering plants. The only choice with that pair uses has risen and contain. Choice D.