【单选题】

What does the professor imply when he says this

A、The 2009 flu outbreak is caused by two separate strains of swine influenzA、
B、The 2009 flu outbreak is caused by influenzaA、virus and human influenzA、
C、The 2009 flu outbreak is due to a combination of several types of influenzA、
D、The 2009 flu outbreak is due to human influenza and avian influenz