verb tense [which prevents or prevent]

[ansonman]

I have made up the sentences below.

(1a) I usually work five shifts a week, which prevents me from having entertainment.

(1b) I usually work five shifts a week, which prevent me from having entertainment.

Some of my non-native friends think (1a) works because the main clause , "I usually .... a week" acts as a single unit, so you need to add an "s" to "prevent".

My other friends don't agree because "prevent" modifies the plural form "five shifts".

What is your opinion? Thank you very much.

[Alan]

Both possible, since either makes sense.