Select the option that best fills in the blank and completes the sentence given below.
The managers have promised to hire the boys once they ...... their degree.
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When talking about future events that are dependent on other actions, use the present simple tense.
The correct form is "complete" in the simple present tense, as it refers to a future event dependent on the boys completing their degree. The sentence should read, "The managers have promised to hire the boys once they complete their degree."