June 2011 LSAT
Section 1
Question 7
Emil-Kunkin on August 1, 2023
B is indeed the correct answer. While the situation you're describing sure is possible, it doesn't show that c doesn't weaken. Showing that the vector of a disease was not present in an outbreak absolutely weakens the idea that the outbreak was caused by that specific disease. While it doesn't completely kill the idea as you noted, we don't need to completely kill the argument to weaken it, we only need to make it less likely. Moreover, there's no real way for B to weaken the argument.