Answer:
The correct option is (C).
Explanation:
Electric current can be direct or alternating.
The direct current flows only in one direction. It is unidirectional.
The alternating current or an C current reverse its direction periodically.
So, the correct statement regarding the alternating current is (c) i.e. Electrons reverse direction periodically.
Answer:
A solenoid field is increased with the increase in the current flow through the coil.
Explanation:
A solenoid is a current carrying coil with a significant number of loops. It works like an electromagnet inside which, a uniform and strong magnetic field is created. The magnetic field outside the solenoid field is nearly zero but not exactly zero.
While experimenting with the solenoid, scientists came to know that the magnetic field generated by the solenoid increases with the increase in the current, flowing through the solenoid.
The magnetic field works strongly around the arena of coil. The field changes its direction only near the ends and gets weak. Else its uniform throughout the loops.
The magnetic field inside the solenoid can be formulated as,

Where,
B = Magnetic field generated by the solenoid
= Constant
I = Current flowing through the loops of the solenoid
Explanation:
It is given that,
Mass of the car, m = 1000 kg
Speed of the car, v = 100 km/h = 27.77 m/s
The coefficient of kinetic friction of the tires, 
Let f is the net force acting on the body due to frictional force, such that,






We know that the acceleration of the car in calculus is given by :
, x is the stopping distance



On solving the above equation, we get, x = 78.69 meters
So, the stopping distance for the car is 78.69 meters. Hence, this is the required solution.