The answer is either C or D..
If net external force acting on the system is zero, momentum is conserved. That means, initial and final momentum are same → total momentum of the system is zero.
"An object at rest will remain at rest unless acted on by an outside force" is the one statement among the following choices given in the question that provides an example of a scientific theory. The correct option among the two options that are given in the question is the first option or option "a". I hope the answer helped you.
Power = (work done) / (time to do the work)
= (21,000 Joules) / (30 seconds)
= (21,000 / 30) joule/sec
= 700 watts
