Michael is a college student. he can either buy a textbook for $100 or save up for a road trip he wants to take during the summer. this illustrates the principle that a. rational people think at the margin. b. people respond to incentives. c. trade can make everyone better off. d. people face trade-offs.

Explanation:

Michael is demonstrating a rational behaviour, because he's trying to make a choice between a necessity and vanity.

So, this thinking process talks about he's ability to rationalise between options.

of what agreement it does not show

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.