The time between two successive meridian crossings of the vernal equinox (or any other fixed point in the celestial sphere) is called day and it is long.

Explanation:

“sidereal day”

“upper median”

It keeps the time, in the two successive upper meridian of the sun during the crossing. Also it is when the star crosses the celestial meridian.

Hope this helps!      

i guess it’s a and b

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.