Explanation
Correct answer: B.
Choice B is the best answer. "Specifically" logically signals that the information in this sentence—that Molina and Rowland’s research laid the foundation for a later treaty—provides specific, precise details elaborating on the previous sentence’s more general claim about the influence of the research.
Why the other choices are wrong
Choice A
Choice A is incorrect because "regardless" illogically signals that the information in this sentence is true despite the previous sentence’s claim about the influence of Molina and Rowland’s research. Instead, this information—that the research laid the foundation for a later treaty—provides specific details elaborating on the previous claim.
Choice C
Choice C is incorrect because "however" illogically signals that the information in this sentence contrasts with the previous sentence’s claim about the influence of Molina and Rowland’s research. Instead, this information—that the research laid the foundation for a later treaty—provides specific details elaborating on the previous claim.
Choice D
Choice D is incorrect because "earlier" illogically signals that the information in this sentence occurred at a time before Molina and Rowland’s research influenced the fight against CFCs. Instead, this information—that the research laid the foundation for a later treaty—provides specific details elaborating on the previous claim about the research’s influence.