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i understand that E is correct, but why B is wrong?

From the stimulus we know that: before taking developmental psychology, experimental design must be completed. In other words, experimental design must be an earlier course than developmental psychology course.

Is there anything wrong with this line of thinking?

Thanks!

You cannot assume one class is easier just because it is a prerequisite. It is not always the case.
Sure, chemistry II might be more difficult than chemistry I since it is a continuation and adds on more concepts.
However, software engineering is different from programming, which means it could be tougher or even simpler.