51. In the spring of 2009, scientists recognized a particular strain of flu virus known as H1N1. This virus is actually a ________ of viruses from pigs, birds and humans.
(A) compartment
(B) coordination
(C) combination
(D) combustion

答案：登入後查看
統計： A(0), B(1), C(0), D(0), E(0) #2768834