The event horizon of a black hole is the point where _____. a. ...

[audie]

The event horizon of a black hole is the point where _____.
 
  a. the mass of the black hole is concentrated in a singularity
 b. the accretion disk around the black hole contacts its surface
 c. the escape velocity equals the speed of light
 d. tidal forces ionize infalling gas
 e. an infalling particle reaches equilibrium with radiation pressure

Question 2

What was Eratosthenes's mathematical contribution to the history of astronomy, even though it was incorrect?
 
  a. He calculated the diameter of Earth by what he learned from travelers' itineraries.
 b. He estimated Earth's revolution rate by measuring the length of shadows casted on the surface.
  c. He determined that all the planets orbited Earth every 24 hrs.
 d. He attempted to calculate Earth's radius by utilizing the position of sunlight.
 e. He believed all of nature was underlain by musical principles.

[amandanbreshears]

Answer to Question 1

c

Answer to Question 2

d