41. What might be the author’s primary purpose of writing this passage? (A) To guarantee that swine flu viruses do not infect humans. (B) To testify that swine flu is the main cause of pigs’ death in the United States. (C)To show the urgent need of inventing specific vaccines to save pigs from being infected. (D) To introduce some basic information about swine flu viruses and how the flu can be prevented.