On what date did the communists take control of cuba? a) july 26, 1956 b) january 1, 1959 c) january 1, 1960 d) december 2, 1956

B is the answer to your problem

there is an explationation of this on a site called crash   course world history 1

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.