The correct answer is A. Each program was based on the knowledge of the previous ones.
The Mercury program was the first American manned space program developed between 1961 and 1963 in the framework of the space race with the Soviet Union.
Based on the knowledge provided by the Mercury program, in 1965 NASA launched the second American manned space program, Gemini, whose main goal was to develop the ability necessary for space encounter and coupling.
These abilities were fundamental for the third program, the Apollo missions, whose main objective was to put the first men on the moon, a goal that was accomplished in 1969.
