A verb form question tests whether a verb is needed to complete a sentence or finish a phrase. Find the subject, then count its main verbs, including any joined by and. Main verbs say what the subject does or is; -ing and to forms can't do that alone. An extra main verb needs a word to join it to the first. If the sentence already has its verbs, read the words before the blank for the form they call for.
Hints
- Hint 1
Find the verbs that say what Rubin did. Read through the word and as you count.
- Hint 2
Look at the phrase directly before the blank. What verb form normally follows that phrase when it names an achievement?
Step-by-step
Count the verbs, then complete the phrase
Step 1Sort the verb forms
Sort the forms of provide by their jobs. A main verb says what the subject does or is: provides and provide can do that, but -ing and to forms can't. The subject here is Vera Rubin, so we need to check whether the sentence already says what she did.
- Step 2
Count Rubin's main verbs
What did Rubin do? She investigated the rotation curves of spiral galaxies and became one of the first, so she already has two main verbs joined by and. Nothing joins another main verb at the blank, which rules out provides and provide. The blank must finish the words after became, not add a new main verb.
- Step 3
Complete the phrase
Which form goes with one of the first? That phrase takes a to form to name what someone did before many others did it. The blank names Rubin's achievement: to provide tells us she supplied evidence for dark matter. Choice D.