A verb form question tests whether a blank needs a main verb, which says what the subject does or is. Read to the period, looking past the phrases and clauses inside a long subject. Check which verbs belong to that subject. A verb showing tense can stand alone; forms with to or -ing need another verb to complete the sentence. Then match the verb to its subject: one thing takes a singular verb, and more than one takes a plural verb.
Hints
- Hint 1
Set aside the opening phrase about the experiments and read the rest through the period. Does the subject of that part already have a main verb?
- Hint 2
Check who or what each verb belongs to: observed, predicted, and acting. Does any of them say what the whole subject does or is?
- Hint 3
In “the pattern of discrepancies,” ask which word the verb must agree with. Do the words after of name the subject itself or tell you more about it?
Step-by-step
Find the missing main verb, then match its subject
Step 1Find the sentence's subject
Set aside During a series of experiments on a low-friction track, which tells when and where the work took place. The words after the comma begin with the pattern of discrepancies, so the pattern is the sentence's subject. To choose the verb form, let's check whether that subject already has a main verb, the verb that says what it does or is.
- Step 2
Check what the other verbs belong to
Do observed, predicted, or acting already give the pattern a main verb? In that physicist Amara Okafor observed, observed belongs to the that clause, a group with its own subject and verb. It says what Okafor did. In the values predicted by an ideal model, predicted tells us that the model predicted the values, while force acting on the cart describes what the force does. The pattern still has no main verb, so the blank must provide one. Having been and to be can't do that job alone, so each would leave a fragment, words that can't stand alone as a sentence.
- Step 3
Match the verb to one pattern
Which word decides whether the verb should be singular or plural? In the pattern of discrepancies, of discrepancies tells what the pattern is about; the plural discrepancies doesn't become the subject. One pattern takes a singular verb, even though the sentence also mentions plural accelerations and values. Replace the subject with it: it was consistent works, while were is for a plural subject. So was gives the pattern a main verb and matches pattern. Choice B.