A verb form question tests whether you can complete a clause, a group of words with a subject and a verb. With describing forms and main verbs among the choices, sort them by job. An -ing or to form can't be a main verb on its own. Find the blank's subject and set aside extra information, because each clause needs its own main verb. If that verb is missing, one thing takes a singular form and more than one takes a plural form.
Hints
- Hint 1
Start at that in the second sentence. Check what part of the sentence it starts, rather than counting every verb in the sentence together.
- Hint 2
Set aside the words between the commas, beginning with despite. Look for a main verb outside that interruption in the part beginning with that.
- Hint 3
What noun does that stand for? Decide whether that noun names one thing or more than one.
Step-by-step
Count, then match
Step 1Find the blank's clause
In the bacterium relies on enzymes that, relies says what the bacterium does. Does that verb also complete the part containing the blank? No, because that starts a separate clause, a subject with its own verb. Each clause needs its own main verb, so we have to check the part beginning with that.
- Step 2
Read past the interruption
Let's set aside despite the lower energy yield of this pathway and the fluctuating concentration of nitrate in the surrounding sediment, the extra information between the commas. Fluctuating describes the concentration, and to sustain ATP production can't be a main verb on its own. No main verb is left in the that clause, so the blank must supply one. A to form such as to remain and an -ing form such as remaining can't be main verbs on their own.
- Step 3
Trace the true subject
What does that stand for? In enzymes that, it points back to enzymes, not to the bacterium. Yield and concentration are inside the extra information, so their singular forms don't control this verb. Enzymes are more than one, which means the clause needs a plural verb.
- Step 4
Match the plural verb
Use they in place of the subject to test the main-verb forms. The singular remains fits it remains, while the plural remain fits they remain. To say that the enzymes stay active long enough to sustain ATP production, we need the plural main verb remain. Choice D.