C.
The force of friction = coefficient of friction * normal force.
Adding the book to the boy increases the normal force and the component of the gravitational force directed down the slide. This in turn increases the force of friction as can be seen by the relationship from the above equation. For a stationary object, the force of static friction is always equal to the force applied (in this case, it is the component of the gravitational force directed down the slide). That means that so long as the boy is not moving and his mass increases, the frictional force is increasing also to balance the increased downward gravitation force directed down the slide.
To solve this problem we will apply the concepts related to Reyleigh's criteria. Here the resolution of the eye is defined as 1.22 times the wavelength over the diameter of the eye. Mathematically this is,
Here,
D is diameter of the eye
The angle that relates the distance between the lights and the distance to the lamp is given by,
For small angle, 
Here,
d = Distance between lights
L = Distance from eye to lamp
For small angle 
Therefore,
Therefore the distance is 5.367km.
Answer:
The mantle exists above the crust of the earth
