An agreement question with several nouns before the blank tests whether a verb matches its true subject. First, sort the choices by number, and check whether the blank needs a main verb or a describing form. A main verb says what the subject does or is; an -ing form needs a helper to do that job. Then find the true subject and cross out describing phrases. One thing takes a singular verb, and more than one takes a plural verb.
Hints
- Hint 1
Read the second sentence to the period. Which part does vibrates belong to, and does it give the words before the blank their main verb?
- Hint 2
Set aside the words between the commas and the phrase beginning with of. What is left as the blank's subject?
- Hint 3
Use it or they in place of the subject you've found. Test which verb forms can follow that word.
Step-by-step
Find the true subject
Step 1Sort the forms and check the blank's job
Sort the verb forms: influence and have influenced go with a plural subject, while influencing can't be a main verb on its own. Does the blank need a main verb? The words how the instrument vibrates form a clause, a group of words with its own subject and verb. The verb vibrates belongs to that clause, so it doesn't give the words before the blank their main verb. The blank needs a main verb, and now we need to find its true subject.
- Step 2
Remove the describing phrases
The phrase of the spruce used for the top plate tells us what has that density, and along with the plate's thickness and curvature adds extra information between commas. Does along with make thickness and curvature part of the subject? No: unlike and, along with introduces extra information, so these properties don't join the subject. The and inside that phrase joins thickness and curvature only within the extra information. Cross out both phrases, and we're left with The density, the true subject.
- Step 3
Match the verb to one density
Is The density one thing or more than one? A density is one property of the spruce, so the subject takes a singular verb. Shrink it to it, and we get it influences how the instrument vibrates, so influences is the form that belongs in the blank. Choice D.