Question

for an experiment to be \fair\

the test and control groups should be
different in as many ways as possible

the test and control groups should be
different in only one way

Explanation:

In a fair experiment, only the variable being tested (independent variable) should differ between the test and control groups. This ensures that any observed differences in the outcome (dependent variable) can be attributed to the independent variable. If multiple factors differ, it becomes impossible to determine which factor caused the change.

Answer:

the test and control groups should be different in only one way