Suppose We Observe a Source of X-Rays That Varies Substantially

Suppose we observe a source of X-rays that varies substantially in brightness over a period of a few days. What can we conclude?

A) The X-ray source must be a quasar.
B) The X-ray source must contain a black hole with an accretion disk.
C) The X-ray source can be no more than a few light-days in diameter.
D) The X-ray source must have a strong, rapidly varying magnetic field.
E) We must seeing the rapid orbit of two stars in a binary system.