Detailed Instructions: In this task, you are given a multiple-choice question and you have to pick the correct option. Answer with option indexes (i.e., "A", "B", "C", and "D").
Problem:In the 60's people watched on TV while for the first time people walked on (A) a high rooftop in a major city (B) a satellite without much air in our solar system (C) the edge of a cliff in the mountains (D) a beach in Brazil
Solution:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
B