cause or causes

[Debra Kirk]

The heartbeat, in addition to other factors, ( cause causes ) the blood to flow.

I chose cause, which is the correct word and what would the rule be for an instance like this?

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[MikeNewYork]

The heartbeat, in addition to other factors, ( cause causes ) the blood to flow.

I chose cause, which is the correct word and what would the rule be for an instance like this?

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The subject is the singular "heartbeat", so you need the singular verb "causes".