Professor Hoffman Was Interested in the Effects of Vitamin B12

Professor Hoffman was interested in the effects of vitamin B12 deprivation on maze learning in rats. One group is given a vitamin B12 injection. The comparison group is
Given a placebo injection. He finds that the vitamin group learns the maze faster. In this study, the independent variable is ________ and the dependent variable is _.

A) speed of learning; type of injection
B) vitamin B12; placebo
C) speed of learning; concentration of sugar
D) type of injection; number of errors
E) type of injection; speed of learning