Explanation
Correct answer: A.
Choice A is the best answer. The second sentence describes a consequence of the system laid out in the first sentence: because air is owned and sold by companies in this world, people have to buy it.
Why the other choices are wrong
Choice B
Choice B is incorrect. This choice uses a transition that means “no matter what happens” or “whatever the situation is,” which doesn’t make sense here. There’s only one situation described in the text: a fictional world in which companies own all the breathable air, forcing people to buy it.
Choice C
Choice C is incorrect. This choice uses a disagreement transition. But this sentence doesn’t disagree with the previous sentence. They both describe the same fictional situation.
Choice D
Choice D is incorrect. This choice uses a transition that indicates a shift back in time, which doesn’t make sense here. Both sentences use the present tense, as they’re describing the same fictional time period.