Choice 'b' is one possible way to state
Newton's second law of motion.
The other choices are meaningless.
Answer:
Explanation:
Force on electron in an electric field E = eE where E is electric field .
acceleration = eE / m where m is mass of electron .
Putting the values
4 x 10⁶ = 1.6 x 10⁻¹⁹ x E / 9.1 x 10⁻³¹
E = 22.75 x 10⁻⁶ N/C
The direction of electric field will be towards west ( opposite to east )
because of negative charge on electron .
Let say the two train cars are of masses
and 
now if the speed of two cars are
and 
then we can say that the momentum of two cars before they collide is given by

here two cars are moving in opposite direction so we can say that the net momentum is subtraction of two cars momentum.
Now since in these two car motion there is no external force on them while they collide
So the momentum of two cars are always conserved.
hence we can say that the final momentum of two cars will be same after collision as it is before collision

Running on sand requires 1.6 times more energy spent than running on hard surface, so the force applied by our foot on sand is less.