Imagine that 1 milliliter of an experimental drug diluted in a saline solution is injected into 20 pregnant mice to determine possible side effects. Which of the following is a suitable control for this experiment?
a. 20 pregnant mice injected with 1 milliliter of saline
b. 20 non-pregnant mice injected with 1 milliliter of the drug
c. 20 male mice injected with 1 milliliter of saline
d. 20 pregnant mice injected with 2 milliliters of the drug
e. 20 male mice injected with 1 milliliter of the drug