What is the direction of the net force acting on the object at position a? hints what is the direction of the net force acting on the object at position a? upward downward to the left to the right the net force is zero?

Looking at the position A, we can actually see that velocity vectors connecting position A and its adjacent positions appear to have similar magnitude and in similar direction. Hence the acceleration is zero and similarly, the net force is also zero.

Net force is zero

i believe the answer would be:

d. violin, clarinet, french horn, and tuba

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.