Hey there, LSFM!
Answer choices (A) and (B) are definitely tricky. My bottom line answer is that they are actually talking about two different sets of bills.
Answer choice (A) says: "most bills that have not been supported by even one leader of a major party have not been passed into law." The subject of this sentence is "bills that have not been supported by even one leader."
Answer choice (B) says: "most bills that have not been passed into law were not supported by even one member of a major party." The subject of this sentence is "bills that have not been passed into law."
So, these choices refer to two different sets of bills. The evidence given in the stimulus is that the leaders of all the major parties oppose the bill. Answer choice (A) gives the historical data, basically an unspoken rule the author is applying, to reach the conclusion that the bill will not pass. So, adding the rule from (A) into the argument:
Premise: The leaders of all the major parties oppose the bill.
Rule: Most bills that have not been supported by even one leader of a major party have passed into law (i.e., the leaders of all the major parties oppose the bill)
Conclusion: Thus, the new agriculture bill will almost surely fail to pass.
This still is a weak argument, because the author is committing a time shift error to use the historical data as a rule to reach a predictive conclusion about the passage of this particular bill. But, answer choice (A) is correct because it provides the rule implicitly, if improperly, applied by the author to arrive at the conclusion based on the evidence given in the premise.
Contrast this with answer choice (B), which does not connect as well with the evidence. It says that if you look at all the bills that didn't pass into law, most of them did not have the support of even one leader of a major party. Answer choice (A) is just looking at the bills that didn't have the support of even one leader, instead of looking at all the bills that didn't pass into law.
Please let me know if I can help you further.
Thanks!
Ron