What is the universe made of? Astronomers face an embarrassing conundrum: they don't know what 95% of the universe is made of.

I think u understand this answers

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation:

nacl or sodium chloride is an ionic compound. this is because an ionic compound pertains to a compound that consists of positively and negatively charged ions. in this case, na (sodium) has a tendency to lose one electron when it reacts with with other atoms, so the resulting sodium ion contains 11 protons and 10 electrons, thus making it positively charged (na+), when chlorine reacts, it usually gains one electron to form an ion called chloride, thus making it negatively charged ( since the ions are oppositely charged, they attract.