Which event most directly resulted from political pressure like that expressed in the excerpt?

The correct answer is Mexican-American War

The Mexican-American War took place between the United States and Mexico from 1846 to 1848, and was motivated by the American ambition over territories that originally belonged to the Mexicans. This conflict was one of the stages of the “westward march,” a process of territorial expansion undertaken by the United States during the nineteenth century.

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.        

i would say b. research typically uses a variety of sources