When creating a campaign experiment, which is a best practice?

Choose two or three metrics that can be used to reliably gauge campaign performance and determine the winner of a particular test.
After the end of an experiment, assess performance over a period of time that includes ramp-up.
Isolate one variable to focus on for each test and use different tests when assessing the impact of more than one change.
As a means of expediting experiments, new ads aren’t subject to the same approval process during time of the experiments.

The correct answer is:

Isolate one variable to focus on for each test and use different tests when assessing the impact of more than one change.