Question

which of the following is/are responsible for giving a galaxy its particular shape?
a) dark matter
b) supermassive black holes
c) dust and gas
d) cosmic microwave background radiation
question 4 (5 points)
why do the terrestrial planets lack hydrogen and helium in their atmospheres?
a) hydrogen and helium are present in rings around the terrestrial planets.
b) hydrogen and helium were blown away by solar wind.
c) hydrogen and helium cant be retained because the planet masses are too small.
d) hydrogen and helium were converted to other compounds as the planets formed

Explanation:

• For the first question:
• Dark matter provides the gravitational framework that helps galaxies maintain and form their shapes. Supermassive black holes are at the centers of galaxies but their direct role in shaping the overall galaxy (beyond the very central regions) is less about the general shape compared to dark matter. Dust and gas are important for star - formation within galaxies but not the primary shapers of the large - scale galaxy structure. Cosmic microwave background radiation is a relic from the early universe and not involved in galaxy - shaping.
• For the second question:
• Terrestrial planets have relatively small masses. The gravitational pull of these planets is not strong enough to retain light gases like hydrogen and helium. Solar wind can have an effect, but the main reason is the inability to retain these gases due to low mass. Hydrogen and helium are not in rings around terrestrial planets, and they were not significantly converted to other compounds during planet formation in a way that would account for their absence in the atmospheres.

Answer:

• First question: A. Dark matter
• Second question: C. Hydrogen and helium can't be retained because the planet masses are too small.