A verb form question with a long subject tests whether the blank needs a main verb. Sort the forms: an -ing or to form can't be a main verb on its own. Then read to the period, separating verbs in smaller parts from the verb for the whole sentence. If that verb is missing, find its whole subject. A regular present-tense verb takes -s for one thing and no -s for more than one, no matter which nouns are nearby.
Hints
- Hint 1
Match each existing verb to its own subject in the second sentence. What do passes, releases, and blur each describe?
- Hint 2
Look at what not only and but also connect. Are they joining separate subjects, or things within the whole Tracking phrase?
- Hint 3
Replace the complete subject with it or they. Decide from what the whole phrase names, rather than the nouns inside it.
Step-by-step
Find the verb for the whole activity
Step 1Sort the forms by their jobs
Sort the forms first: allowing and to allow can't be main verbs on their own. A main verb says what the subject does or is, so we need to find that verb in the sentence beginning with Tracking. The verb about the laser pulse belongs to the first sentence, so start the count in the sentence with the blank.
- Step 2
Set aside the verbs in smaller parts
Do the verbs inside the long sentence already do that job? In how rapidly that energy passes and how efficiently the lattice releases it, passes goes with energy and releases goes with the lattice, not with tracking. Later, that slower measurements blur together describes the heat-transfer stages, so blur belongs to measurements. None says what tracking does for physicists, which means the blank still needs a main verb.
- Step 3
Decide whether the whole subject is singular
Does not only how rapidly paired with but also how efficiently make the subject plural? Those words connect two things being tracked, not two separate subjects. The subject is the whole phrase beginning with Tracking, and an -ing phrase used as a subject names one activity, even if it includes several things. So we can replace the complete subject with it rather than they.
- Step 4
Test the main verb with it
Replace Tracking and the words saying what is tracked with it, then test the present-tense forms. It allow doesn't agree, while it allows does, because the singular form of this verb ends in -s. Choice B.