A is incorrect: Changing the model along with the prompt would introduce multiple variables, making it impossible to determine which change caused the observed results.

B is correct: For an accurate assessment of a prompt's effect, it is essential to ensure that the prompt is the sole element altered throughout the experiment, isolating its impact.

C is incorrect: Using different API keys does not inherently affect the validity of A/B testing unless it leads to different environments or models, which would violate the single-variable rule.

D is incorrect: Varying the test times introduces an uncontrolled variable, which can invalidate the comparison between prompts.