When performing email A/B tests, _______.
1) testing one variable at a time is recommended, because segmentation renders multivariate testing impossible
2) testing one variable at a time is recommended, because it allows you to isolate the effect of each variable
3) testing multiple variables at a time is recommended, because it allows you to view a range of effects and possibilities in your email marketing
4) testing multiple variables at a time is recommended, because it allows you to isolate the effect of each variable