Atype of investigation that allows scientists to control variables is an

There are three types of scientific investigations:

1.Descriptive investigation.

2.Comparative investigation

3.Experimental investigation

Among these, the experimental investigation is a type of investigation, which involves the control of various variable parameters. In Descriptive investigation, only the description of the sample being tested is done, it does not involve controls on the variable. In comparative investigation, a comparative study is done between two or more substances, where the circumstances remain the same, and there are no variables, which are under control.  

answer; i believe the answer is hydraulic action;