I had this one down to C and D and ultimately went with C. I saw C an an alternate explanation but would C have been eliminated because the increase was not specifically mentioned a well as the FM? Do both A and B both have to be specifically mentioned and not implied or assumed? Also, I get that C is incorrect because C-> A&B (Other M -> FM & Increase isn't possible due to no assumption in the stimulus that FM caused the increase but is instead the conclusion, so FM->Increase (A->B) was never a possibility. I ultimately eliminated D because I interpreted the AC as saying there was no increase in the algae population because there was no increase in the amount of shells left behind. I took that as meaning constant but didn't go further and see that it meant that we should see more shells if there was an increase, thus weakening the argument. Hopefully this all makes sense for you guys.
https://classic.7sage.com/lsat_explanations/lsat-19-section-2-question-04/