Final answer:
Eratosthenes measured the size of the Earth by observing the angle at which the Sun's rays hit its surface. If the world had been flat, Eratosthenes would not have recorded any angle.
Step-by-step explanation:
Eratosthenes was an ancient Greek mathematician and geographer who accurately estimated the size of the Earth. He measured the size of Earth by observing the angle at which the Sun's rays hit our planet's surface. In Alexandria, Egypt, at noon on the first day of summer, Eratosthenes observed the Sun making an angle of 7° with the vertical. He reasoned that the curvature of the round Earth meant that "straight up" was not the same in Alexandria and Syene. By measuring this angle and knowing the distance between the two cities, he was able to estimate the circumference of the Earth. If the world had been flat, Eratosthenes would not have recorded any angle since the Sun's rays would not have differed in angle at different locations. Therefore, the answer to the question is d. 0 degrees.