How Did Eratosthenes Estimate the Size of Earth

How did Eratosthenes estimate the size of Earth?

A) They used the height of sailing ships as they sailed over the horizon to estimate curvature of Earth, which allowed estimation of its size.
B) They didn't; everyone thought the world was flat until Columbus' time.
C) They measured the angle of the Sun above the horizon at midday in two places with different latitude and used the angle to estimate the circumference of Earth.
D) They measured the altitude of mountains in Greece, then used the distance they were lost from sight to estimate the curvature of Earth, which allowed estimation of the its size.