In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.

Question: When did construction begin on the Space Station launched in the 1970's based on a design of the Saturn IB S-IVB upper stage called Skylab A? Passage:In the 1970s, the Skylab Space Station is launched, but apparently as a wet workshop design that is based on the Saturn IB S-IVB upper stage called Skylab A. The Saturn V that might have launched Skylab in our timeline instead launches Skylab B, a lunar orbit space station unofficially named "Moonlab", also a wet workshop based on the S-IVB. The Apollo-Soyuz Test Project is instead a series of visits by the Apollo Command/Service Module to Salyut space stations, and Soyuz missions to both Skylab and Moonlab. To facilitate the latter, the Soviets finally finish work on their N-1. The Skylab/Moonlab programs lead to improvements in the design of the Apollo Command/Service Module. A Block III CSM is produced using battery power in place of fuel cells, followed by the Block IV and V, which have a degree of reusability (modular construction and resistance to salt water corrosion). Also chronicled is the development of the experimental 'Mars Excursion Module' by small aerospace firm Columbia Aviation as it struggles against larger rival contractors of NASA and its engineers working painstakingly against the technical challenges of a working and reliable Mars lander.
Links: a. Skylab b. S-IVB c. S-IVB d. ApolloSoyuz Test Project
a