Why are experimental investigations prove to be the best type of scientific investigation to demonstrate cause-and-effect relationships?

Experimental investigations are deemed the most important and the best type of scientific investigation because we can study the cause - and - effect relationship between different variables in a controlled environment. This includes conducting test to test hypothesis and validity of the claims. 

I believe the answer is: because we can study the cause - and - effect relationship between different variables in a controlled environment.

In a controlled environment, we can freely change the factors that might influence the result whenever we want, in order to cross check whether a certain factor would resulted in the desired effect or not. This includes conducting test to test hypothesis and validity of the claims.

1a

Explanation: