What Is the Most Effective Way to Minimize the Effect

What is the most effective way to minimize the effect of a large number of unknown extraneous variables?

A) Match the groups by hand.
B) Randomly assign subjects to treatment and control group.
C) Use an experimental design.
D) Place subjects into groups according to the extraneous variables they possess.
E) Allow subjects to choose the groups to which they want to belong.
F) Use a large sample.