Questions on subject-verb agreement

Subjects and verbs must always agree in English. The basic rule for subject-verb agreement is that singular subjects take singular verbs, and plural subjects take plural verbs. For this reason, you must always identify the real subject in order to choose the correct verb form.

Let’s look at the first example:
1. The number of the students who register for the course __________ .
a) has been rising
b) have been rising
c) are rising
d) risen
The real subject of the above statement is “The number” which takes a singular verb as it is a singular noun in this sentence. The only singular verb among the given choices is a) has been rising.

Look at the next example:
2. All of the tourists’ luggage _______ at the concierge.
    a) leaving
    b) are left
    c) has been left
    d) were left
The subject of the statement is “luggage” which is an uncountable noun, so it is singular. The answer is, therefore, c) has been left.