I think pretty interesting discussion has gone in this thread, with some of the members making really elaborate and explanatory posts.
However, I think the issue at hand can be simplified further. So, let me try my bit at it.
Clearly, the only confusion is between option C and option E. The only difference between option C and option E is that option E makes the entering year later than 1990.
So, let's just focus on this part:
Let's see the relevant part of the passage:
"In the Business program at a university, all candidates for the MBA in International Business who entered the program after 1990 took a seminar on Microeconomics"
and the relevant part of question stem
"If a student in the Business program described above took a seminar on Microeconomics"
So, the passage says that all students (or candidates, let's use the same terms) who entered program after 1990 took Microeconomics
Now consider, if I, during my MBA, took a course on Microeconomics; can you say that I entered the program only after 1990?
If your answer to this question is yes, then my question is that what makes you think that I could not have entered before 1990? Clearly, all students who entered after 1990 had to take Microeconomics but it doesn't mean that students who entered before 1990 could not take Microeconomics.
If students who entered before 1990 could indeed take Microeconomics, then I could be one of those students. Isn't it?
Let's consider an analogy here:
All students who entered the school in a red dress got a tight slap on their face.
Now, if I say that I got a tight slap on my face, can you say that I wore a red dress to the school?
No, you can't say that. Because a red dress ensures a slap but the school authorities could, at their whims and fancies, slap anyone in any dress
We are only given that a person in red dress won't be spared but we can't say about others.
Similarly, if a person took Microeconomics (a slap), we can't say he entered after 1990 (wore a red dress).
Hope this helps
If anyone has doubts, please feel free to ask.
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