Which of the Following Statements Concerning Influenza Is Incorrect

Which of the following statements concerning influenza is incorrect?

A) The elderly and patients with chronic health problems are at greater risk for complications.
B) Type C influenza is milder than the other types.
C) One type of influenza mutation is called antigenic drift, which refers to an abrupt change in the virus strain.
D) Several influenza pandemics have occurred in the past.