Choose the correct form of the verb to complete the sentence. Yuri Gagarin ____ the first astronaut to travel to outer space.

Rationale:

• The sentence describes a completed action in the past.
• 'Yuri Gagarin' is a singular subject.
• 'became' is the past tense of 'become' and fits grammatically and contextually.
• 'been' is part of a perfect tense and requires an auxiliary verb like 'has' or 'had'.
• 'is' is present tense and does not fit the past event.
• 'become' is the base form or present tense and does not fit the past event.

Answer: became