The first confirmed black hole, Cygnus X-1, was regarded as proof that black holes exist because it
a. is a compact object in a binary system
b. emits X-rays from a hot accretion disk
c. is accreting matter from a main-sequence companion
d. has a period of 5.6 days
e. has a mass of ten solar masses
Question 2
Kepler's laws of planetary motion were _____ because he could not provide an explanation for his conclusions.
a. unnatural
b. empirical
c. hypothetical
d. principles
e. assumptions