What distinguishes push-ups from other chest exercises such as the bench press and weight flies?
a.
the bench press also exercises the legs.
b.
push-ups are a variable resistance exercise.
c.
push-ups do not require any weights or machines.
d.
push-ups are not actually chest exercises.

Push-ups are an exercise which target the muscles in the chest, abdomen, shoulders and the tricep. This exercise differs from other chest exercises because C. push-ups do not require any weights or machines. Push ups are done using one's own bodyweight. They can be done at home or in the gym, meaning they are a convinient exercise.

Well if you pick a machine to work on and that's gonna be your exercise if you don't stick to that machine your muscle strength your muscle mass rather will change because you haven't been doing it you change something in your exercise routine which changes your body's form of dealing with things for example you might become weaker or stronger depending on what kind of exercises you do in whether or not the machine you picked actually your muscles grow.