The Subjects in the Experimental Group Receive the Experimental Drug,while

The subjects in the experimental group receive the experimental drug,while the subjects in the control group receive a placebo.Although the experimenter knows which subjects are in the
Experimental group and which are in the control group,the subjects themselves do not know
Who is in which group or who received the real drug and who the placebo.This experimenter
Is using

A) a double-blind study to control for researcher bias.
B) a single-blind study to control for research participant bias.
C) random selection to control the extraneous variables.
D) random assignment to control the dependent variables.