An experiment can show: OA. how changing a variable can affect behavior.
O B. how observations can be valid.
OC. wording bias.
OD. interviewer bias.

A. how changing a variable can affect behavior.

Explanation:

An experiment can be defined as an investigation which typically involves the process of manipulating an independent variable (the cause) in order to be able to determine or measure the dependent variable (the effect).

Hence, an experiment can show how changing a variable can affect behavior.

This ultimately implies that, an experiment can be used by scientists to show or demonstrate how a condition causes or gives rise to another i.e cause and effect, influence, behavior, etc in a sample.

when you rub, you also apply pressure, that pressure can urge the poop to come out. i think that when your rubbing, you also might be slightly breaking it up and that can further fill the stomach.