A verb form question tests whether the blank needs a main verb or a describing form. Find the subject, who or what the sentence is about, then read to the period and count main verbs. A main verb says what the subject does or is; if none appears, the blank must supply one. Describing forms can't stand alone as main verbs. Then check the subject's number: singular subjects take singular verbs, and plural subjects take plural verbs.
Hints
- Hint 1
Read through the phrase beginning with motivating and check whether the sentence already has a main verb.
- Hint 2
Look at the full list before the blank. Where does each item begin and end?
- Hint 3
Replace the whole subject with it or they, then check which verb form goes with that pronoun.
Step-by-step
Find the main verb, then match the subject
Step 1Read through the ending
Read past the blank to motivating utilities to fund large-scale pilot projects. Does this ending supply the main verb, the verb that says what the subject does or is? An -ing form without a helper such as is only describes, and the to form after it can't be a main verb either. No main verb appears elsewhere, so the blank must supply one.
- Step 2
Set aside the describing forms
Which forms can fill that job? The phrase by making tells how something happens, while having made describes something already done. Neither supplies a main verb, so the sentence would be a fragment, a group of words that can't stand alone as a sentence.
- Step 3
Find the whole subject
Who or what causes the cables to be an attractive option? Look before the blank: Unique mechanical durability, negligible energy loss over long distances, and compatibility. Those are three features joined by and, so the full subject is plural. The phrase with existing fiber infrastructure tells us what the third feature is compatible with. The words superconducting power cables come after the blank and name what the features affect.
- Step 4
Match the verb to the plural subject
Shrink the subject to they and test the present-tense forms. A present-tense verb takes -s with it but not with they, so we say it makes and they make. The form makes would match one feature, but make agrees with the plural subject. Choice D.