I know it's common for beginners to (mistakenly) think there are multiple correct answers to a question, but I just can't see how there's only one correct answer in this case. Any input would be greatly appreciated.

Question: PT114.S2.Q9

Complete stimulus: During the three months before and the three months after a major earthquake in California, students at a college there happened to be keeping a record of their dreams. After experiencing the earthquake, half of the students reported dreaming about earthquakes. During the same six months, a group of college students in Ontario who had never experienced an earthquake also recorded their dreams. Almost none of the students in Ontario reported dreaming about earthquakes. So it is clear that experiencing an earthquake can cause people to dream about earthquakes.

Complete question stem: Which one of the following, if true, most strengthens the argument?

Complete answer choices:

A: Before the California earthquake, no more of the students in California than of those in Ontario recorded dreams about earthquakes.

B: The students in California were members of a class studying dreams and dream recollection, but the students in Ontario were not.

C: Before they started keeping records of their dreams, many of the students in California had experienced at least one earthquake.

D: The students in Ontario reported having more dreams overall, per student, than the students in California did.

E: The students in Ontario who reported having dreams about earthquakes recorded the dreams as having occurred after the California earthquake.

Answer choice A is marked as correct, and answer choice D is marked as incorrect.

My reasoning: Answer choice D strengthens the argument by eliminating an alternate explanation: California students had more dreams about earthquakes because they have more dreams in general. Therefore, an earthquake occurring in California would not be the reason why half of the students reported having dreams about earthquakes.

Thanks for the help!