Most serious students are happy students, and most serious students go to graduate school. Furthermore, all students ...

I understand why B was correct, but I am curious as to why D was incorrect. In the explanation the answer gives for why D is incorrect, it says it is because you cannot take the flip a most statement, which I understand. However, in the lesson, we learned that when you have two most statements, they create a some statement. And you can flip a some statement. So since the D is actually the reversal of a some statement, why wouldn't it be correct?