A verb form question with a that clause tests whether the verb completes its clause and matches its subject. Read around the blank and sort the forms: an -ing form needs a helper to be a main verb. Then find the blank's own subject, because every clause needs a main verb. If that is the subject, find the noun it refers to. In the present tense, one thing takes a verb with -s or is, while more than one takes a verb without -s or are.
Hints
- Hint 1
Find the main verb for the population. Which words already say what the population has done?
- Hint 2
Look at the words right before the blank. Does that start a part that needs its own subject and main verb?
- Hint 3
Ask what that stands for. Is that noun one thing or more than one?
Step-by-step
Find the blank's clause and subject
Step 1Find the main verb already there
Find the population of river otters and its main verb, has increased, which says what the population has done. For the blank, we need to decide between a main verb and a form that only describes. A bare -ing form can't be a main verb, but adding a helper such as are makes it one. The population already has its main verb, but does the blank belong to the same part of the sentence?
- Step 2
Give the new clause its own verb
The word that in a trend that changes the blank's job. What does it start? It begins a clause, a part of the sentence with its own subject and verb, and that is the subject here. This clause needs its own main verb, because the population's verb is in another clause and the words after the blank contain no verb. Demonstrating has no helper, so it would leave the that clause without a main verb.
- Step 3
Match the verb to one trend
Now ask what that stands for: it points to a trend, not to the river otters. One trend is singular, which means its present tense verb takes -s. Demonstrate and are demonstrating go with they, so they don't match this subject. Demonstrates goes with it and gives the that clause the main verb it needs. Choice C.