How Did Scientists Realize They'd Been Stimulating the Part of a Rats

How did scientists realize they'd been stimulating the part of a rats brain in an area that made the rat "feel" good?

A) The rat kept behaving in such a way as to repeat the stimulus over and over, no matter what other activities were offered.
B) The rat would twitch its whiskers in response to the stimulus, which is how rats communicate to other rats that something is good.
C) Electrical stimulation in any part of a rats brain caused it to try to behave in such way as to repeat the stimulation, so it must have "felt" good.
D) When rats were given a choice between the stimulus and a piece of their favorite food, they couldn't decide what to do and so did nothing.
E) The rats were trained to push a lever when they "felt" good and they did this in response to particular brain stimulations.