A/B test generalizability

Listed below are a set of decisions that could be made when designing an A/B test. Identify the decision that would not cause an issue in generalizing the test results to the overall user population.

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Possible Answers

Assigning users to the Test or Variant group based on their signup year.
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Using a hash of the randomly assigned user id to determine user groupings.
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Randomly assigning users within one country to different groups.
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Allowing users to change groups every time they use the service or software.
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