In urban areas, the cell towers become more numerous and the cell signals transmitted by the cell towers are than those in less populated areas.

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.        

answer; /// all of the above;

answer; severe;