A is incorrect because there are many people involved in "The Adventure of the Six Napoleons" that commit crimes.
B is incorrect because in the first story, he takes the case at the urging of a professor at the university and in the second story, he takes the case and works with Lestrade.
C is correct because in "The Adventure of the Three Students" no one is hurt, but someone was murdered in the other story.
D is correct because in "The Adventure of the Three Students" a serious crime isn't really committed and so there was no reason for the police to be involved. However, in the second story, Lestrade asks for Holmes' help and they work together.
