For questions 17-21, write the letter of the definition that best matches the term or phrase in the space provided : 17. Quasar
18. Cosmic background radiation
19. Nova
20. Protostar
21. Pulsar
A. A shrinking region of space flattened into a disk with a central concentration of matter.
B. An object that produces energy at a high rate at the center of some galaxies.
C. A neutron star that emits radio waves.
D. Possible evidence of the big bang.
E. A large, bright explosion of a white dwarf caused by pressure building up

(Which Letter should go on what box up top? Please help! <3)