Common Trap #1: Interrupting Phrases
Interrupting phrases are phrases that separate the subject from the verb. Such phrases make locating the subject and determining whether the verb should be singular or plural more difficult. There are specific types of interrupting phrases and we'll take a closer look at a few of them. You don't know need to know all of the specific grammar terms, but it's important to recognize how they influence subject-verb agreement questions.
Example:

This trend, combined with the increasing accessibility of information via the Internet, has led some to claim that librarianship is in decline as a profession. As public libraries adapt to rapid technological advances in information distribution, librarians’ roles are actually expanding.

A) NO CHANGE
B) have
C) which have
D) which has