I looked up the question and got D- a vacuum
What are the two sets? I only see the two answers?
Explanation:
Seismic refraction is the bending of the seismic waves as they pass geological layers of the earth due to different densities. This is especially true for Primary waves because they can pass through all the layers of the earth both liquid and solid.
Seismic reflectivity is the bouncing back of seismic waves at a boundary of geological layers due to different densities or subsurface formation. This is especially true for secondary waves that are unable to pass through liquid layers of the earth like the outer core and mantle. When they reach the boundary of these layers they bounce back towards the earth's surface.
These properties of P and S waves are used to ‘auscultate’ the epicenter of an earthquake by triangulation.
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Explanation:
The centripetal force acting on a particle is given by :
Since, 
Case 1.
If 
Since, 
So, 
So, the force becomes,
F is independent of r
Case 2.
If 
So, 
So, the force is inversely proportional to the square of radius.
Case 3.
If T is independent of r, the force will be directly proportional to the radius of orbit.
Hence, this is the required solution.
