Question

why is it important to have a controlled experiment? to determine the cause of observed changes to make the experiment more challenging to test multiple variables at once

Explanation:

A controlled experiment keeps all variables constant except the one being tested. This eliminates confounding factors, allowing researchers to link observed changes directly to the manipulated variable, establishing a causal relationship. The other options are incorrect: controlled experiments simplify (not complicate) testing, and they isolate a single variable, not test multiple at once.

Answer:

to determine the cause of observed changes