Spiral, elliptical, irregular, and barred spiral are descriptions of different shapes of a. stars b. galaxies c. solar systems d. orbits

B. galaxies

Explanation:

Galaxies are clusters of stars, gas and dark matter. They are classified based on the shape they form. There are three main categories: Spiral, elliptical, and irregular. As their named suggests, that is what they look like.

Barred spiral galaxies are a subcategory of spiral galaxies. The nucleus of the galaxy (Center) a bar material is formed and arms branch out from the ends of it.

Some galaxies on the other hand, do not fit the main categories.

Attached is a picture of the common and uncommon categories of galaxies.

the skeletons of most mammals reach a certain size and then stop growing. however, many animals, including some mammals, keep growing throughout their lives. kangaroos, for example, just keep growing and growing until they die. also, the longer an animal lives the greater the chance it might catch some deadly disease.

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.