1. quartz crystals normally used in quartz clocks etc. is chemically a. silicon dioxide b. germanium oxide c. a mixture of germanium oxide and silicon dioxide d. sodium silicate 2. which of the gas is not known as green house gas? a. methane b. nitrous oxide c. carbon dioxide d. hydrogen

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium