An Example of a Randomized Controlled Experiment Is When

An example of a randomized controlled experiment is when

A) households receive a tax rebate in one year but not the other.
B) one U.S.state increases minimum wages and an adjacent state does not, and employment differences are observed.
C) random variables are controlled for by holding constant other factors.
D) some 5th graders in a specific elementary school are allowed to use computers at school while others are not, and their end-of-year performance is compared
Holding constant other factors.