Mark's ability to distinguish a trumpet and a trombone or his mother's voice from his sister's voice is most likely due to the of these stimuli.

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.        

dude you should crop your image so only the question shows. but   can you explain it better