Word groups—often prepositional phrases—sometimes come between the subject and the verb. Such word groups, usually modifying the subject, may contain a noun that at first appears to be the subject. By mentally stripping away such modifiers, you can isolate the noun that is in fact the subject.
The subject is levels, not pollution. Strip away the phrase of air pollution to hear the correct verb: levels cause.
The subject is slaughter, not pandas or pelts.
NOTE:Phrases beginning with the prepositions as well as, in addition to, accompanied by, together with, and along with do not make a singular subject plural.
To emphasize that two people were shot, the writer could use and instead: The governor and his press secretary were shot.
Subject-verb agreement at a glance
When to use the -s (or -es) form of a present-tense verb
Exercises:
Subject-verb agreement 1
Subject-verb agreement 2
Subject-verb agreement 3
Subject-verb agreement 4