An advertiser is running a campaign on a new advertising platform where they will spend $100,000 over the course of four weeks with the goal of driving incremental purchases. The campaign targets men, ages 18-34 and does not hold out any users from seeing the advertiser's ads. Typically, the advertising platform sees that campaigns at this spend level reach about 75% of the target audience. The analytics team at the advertising platform recommends measuring the effect of the campaign by measuring the conversions driven by all users who saw an ad versus all users who did not see an ad.

What is the primary limitation of this approach?

A. It can end up with different sample sizes, making the test and control group uncomparable

B. It can introduce a degree of imbalance across the test and control groups

C. It is not possible to calculate the statistical significance of this test

D. It does not control for unobservable effects impacting the test group