Which of the following statements about motion are true? select all that apply.
a. if an object is changing direction, a force must be acting on it.
b. if an object is changing speed, a force may or may not be acting on it.
c. an object can move to the right only if a force is pushing or pulling it to the right.
d. an object that is moving through outer space far from any other objects will keep moving in a straight line indefinitely.

A, C, D

Explanation:

Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.

and according to Newton's 4th law: An object that is at rest will stay at rest unless a force acts upon it. An object that is in motion will not change its velocity unless a force acts upon it.

the part of the wave he is travelling on is the crest