Your question is Set Up a Valid A/B Test. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You work on a product experience that your team wants to improve with an A/B test. The proposed change is small enough that the team is debating how to define the control group and whether the test would be internally valid.
How would you set up the experiment, including the control group, and how would you protect internal validity? Explain the choices you would make and the main threats you would guard against.