Biology

The correct answer is: tests only one variable at a time.

Here's why:

1.In a controlled experiment, researchers manipulate one independent variable (the variable they are testing) while keeping all other variables constant (controlled) to isolate the effect of that one variable on the dependent variable (the outcome being measured). This ensures that the results are due to the manipulation of the independent variable alone.

Let’s break down the other options:

B.Requires multiple dependent variables: While some experiments may have more than one dependent variable, a controlled experiment typically focuses on a single dependent varia...