B. An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object slows down. You can't tell. We are so used to talking about forces that we often forget that every time one object affects another it is in turn affected.
Physicists talk about interactions .
The idea of interaction is familiar to us. Two people are said to interact if they influence each others behavior. The key idea here is that interaction is a mutual process. The influence can depend on many different properties of the two objects. Force is the quantity that describes how a single particle is affected by the different interactions it participates in.