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

The correct answer is:

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