Explanation

Eratosthenes measured the Earth's circumference around 240 BCE by comparing the noon sun's angle at Alexandria, where a stick cast a shadow, with Syene (modern Aswan), where the sun shone straight down a well on the summer solstice. From the angular difference and the inter-city distance he computed a remarkably accurate circumference. Troy and Thebes were not part of this experiment. Hence option A is correct.