A verb form question with a long subject tests whether the blank needs a main verb and which form matches the subject. Sort the forms: an -ing form needs a helper to be a main verb. Then read to the period, setting aside verbs in parts starting with since or because. Find the whole subject: one thing takes a singular verb, and more than one takes a plural verb. Nearby nouns can distract you from what the subject actually names.
Hints
- Hint 1
Read the sentence beginning with Building through its period. Check which words could be its main verb, and ask whether the verb after since belongs to the same subject as the blank.
- Hint 2
Look for the whole subject of the blank, rather than stopping at a noun inside it. Could you replace that whole subject with it or they?
Step-by-step
Find the main verb, then match its subject
Step 1Check whether a main verb is missing
Sort the choices into forms that can be main verbs and an -ing form that can't be one without a helper. Read from Building a chronological map to the period: does this sentence already have a main verb? The signatures are described as preserved, and since high-altitude winds can transport ash thousands of kilometers tells us what the winds do. Neither gives a main verb for building the map, so the blank needs a main verb.
- Step 2
Find what needs calibration
What needs careful calibration: the map, the ice cores, or building the map? The subject is the whole phrase beginning Building a chronological map of eruptions, which names the activity of making that map. The words from the chemical signatures preserved in dozens of ice cores tell us what researchers use to build it, so the plural nouns inside that phrase don't decide the verb. One activity is a singular subject, even when it involves many eruptions or cores.
- Step 3
Choose a singular main verb
Which form gives that singular subject a main verb? Require and have required are plural main-verb forms, and requiring is an -ing form without a helper. Shrink the long subject to it: it requires careful calibration works because requires is a singular main verb. Choice A.