By looking closely at how stars and galaxies are moving away from other, what have astronomers been able to calculate

a. the age of the universe only

b. the rate at which the universe is expanding only

c. the age of the universe and the rate at which the universe is expanding

Which of the following correctly describes what happens when an atomic bomb explodes? small pieces of fissionable material are joined and form a body with a mass greater than the critical mass, the relative number of neutrons escaping decreases, and a chain reaction and explosion result. large pieces of fissionable matter are brought together quickly and form a body with a mass smaller than the critical mass, the relative number of escaping neutrons increases, and a chain reaction and explosion result.