Fill in the blank: When performing email A$/$B tests,

testing one variable at a time is recommended, because segmentation renders multivariate testing impossible

testing one variable at a time is recommended, because it allows you to isolate the effect of each variable

testing multiple variables at a time is recommended, because it allows you to view a range of effects and possibilities in your email marketing

testing multiple variables at a time is recommended, because it allows you to isolate the effect of each variable