Piano strings vibrate at their natural frequency

A. True
B. False

May someone me with these science questions

Clyde and marilyn are riding a roller coaster. during which section(s) of the track is their potential energy converted to kinetic energy? a. from point b to point c only b. from point b to point d only c. from point a to point b only d. from point a to point b and from point c to point d

Imagine that you push on a large rock. at what point does your effort change the rock’s mechanical energy?