A verb agreement question tests whether a verb matches its subject, the thing doing or being something. Sort the verb forms by number, and check whether the sentence needs a main verb. An -ing form can't be that verb without a helper such as is. To find the true subject, ask who or what does the verb's work and set describing phrases aside. Use it for one thing and they for more than one to test which verb form agrees.
Hints
- Hint 1
Read the second sentence to the period and check whether it already has a main verb, a verb that says what something does or is.
- Hint 2
Ask what lets the material bend. Find the subject that answers that question, then check whether it names one thing or more than one.
Step-by-step
Find the true subject
Step 1Sort the forms and check for a main verb
Start with number: allow and have allowed go with more than one thing. Does the second sentence have a main verb, a verb that says what something does or is? Read through the material to bend and then return to its original shape: the sentence has no main verb elsewhere, so the blank needs to be a main verb. Allowing can't do that without a helper, so we can set it aside.
- Step 2
Find what lets the material bend
What lets the material bend and return to its shape? The phrase of long molecular chains tells us what the network is made of, so let's set it aside. That leaves a network as the true subject, the thing doing the allowing. A network is one thing, even though it contains many chains.
- Step 3
Match the verb to one network
Replace a network with it to test the number. We say it allows; allow and have allowed instead go with they. So allows is the singular main verb the sentence needs. Choice B.