18. Which of the following fusion reactions cannot produce energy to keep a star from collapsing in on itself?
a. hydrogen into helium
b. helium into carbon
c. silicon into iron
d. iron into tellurium

19. How big is a neutron star?
a. about the size of San Francisco (about 12 kilometers across)
b. about the size of the Earth
c. about the size of the Sun
d. about the size of the orbit of Mars

20. Why are black holes black?
a. because they are white dwarfs that have lost all their remnant heat
b. because they are so hot they only emit light in the ultraviolet
c. because their escape velocity is greater than the speed of light, so any light they emit can't escape
d. because they are moving away from us so quickly, their light is redshifted out of the visible range