C is the correct answer. the daily times article
azithromycin would be C: an antibiotic.
(hope this helps ^^)
I'm not sure since I can't read the passage, but I think D is least important.
Based on the given sentence above, I can say that it is a compound sentence. What makes this sentence compound is that, it consists of two independent clauses and are connected by a coordinating conjunction "so". Therefore, the answer for this would be the last option: two independent clauses.