What Does a Researcher Do When Conducting an Experiment

Multiple Choice

What does a researcher do when conducting an experiment

A)  in-depth investigation of an individual subject through detailed documentation 
B)  observation of behaviour as it occurs in its natural environment 
C)  systematic observation or measurement of two variables to see whether there is an association between them 
D)  manipulation of a variable under carefully controlled conditions and observation of whether there are changes in a second variable as a result