Ethicist: Every moral action is the keeping of an agreement, and keeping an agreement is nothing more than an act of...

I was having difficulty answering this question but I got it right. I was able to figure out the answer by realizing there was a missing premise to get to the final answer because its a jump from KA-some-not Ma to SMB-some-not MA. I realized it was relying on the reverse of KA->SMB which is SMB-some-KA to make the connection to the conclusion. After that I just looked for the answer choice that had the same this as the given statement. Is this a right way of thinking or did I just get lucky this time?