Answer: Option (a) is the correct answer.
Explanation:
AC current stands for alternating current. In an alternating current there is periodically reverse movement of current.
This means electrons move in one direction for a certain time and then they reverse their movement for the same time period and it continues in the back and forth position.
Thus, we can conclude that the statement electrons move back and forth is true regarding AC current.
Explanation:
Newton's second law says that when a constant force acts on a massive body, it causes it to accelerate.
At the ground the ball will always have velocity along the direction of gravity. If upward motion is taken positive it will always have negative velocity at the ground because, if the ball was given an initial upward velocity then gravity will decelerate it and bring it down with a negative final velocity. If the ball is given an initial downward velocity then the ball will be further accelerated by gravity in the downward direction only, again maintaining negative direction. The magnitude however in both cases will be different. the final velocity at the ground will have higher magnitude in case of elevator moving downwards.
The initial kinetic energy of the boat and its rider is

After Sam stops it, the final kinetic energy of the boat+rider is

because its final velocity is zero.
For the law of conservation of energy, the work done by Sam is the variation of kinetic energy of the system:

where the negative sign is due to the fact that the force Sam is applying goes against the direction of motion of the boat.