【单选题】

Questions 57 to 61 are based on the following passage.

Though it now seems merely an episode in the last year of World War I, the influenza pandemic of the autumn of 1918 was one of the three greatest outbreaks of disease in history. Only the plague of Justinian and theBlackDeath compare with it.A、quarter of the world’s population was affected; all in all, it killed 22 million people, almost twice as many as were killed in the war itself. In India, more people died from influenza in a few months than had died from cholera in twenty years. In the United States, half a million people dieD、
Through centuries, the course of epidemics has run from east to west. The 1918 influenza epidemic followed this pattern, reachingAmerica last. Traditionally,Asia has been the matrix of disease, almost as though there existed, in the vastness of Mongolia, a permanent focus of infection which would erupt periodically into the rest of the worlD、Some doctors maintained that the influenza was introduced byChinese labor battalions that landed on the coast of France. Some attributed it to Russian soldiers arriving from Vladivostock. Others thought it might have developed in Spain from an earlier bronchitis which was so prevalent during the spring that the name "Spanish" was given to the autumn influenzA、There was even one tenuous theory that the disease sprang into being in an isolated Georgia training camp during the winter of 1917 and migrated westward until it had circumnavigated the earth.
Influenza is still a mysterious disease. No one yet knows whether it is one virus or several, why it occurs in cycle, or how and where it lies dormant between epidemics. There are theories of weather, theories of the wearing off of group immunity, and even a theory of determination by economic circumstances. The most generally held current explanation is, however, that a pandemic like that of 1918 arises when a new and explosive strain of virus develops through a spontaneous process of mutation or renewal.
Through the centuries, the course of epidemics has been ______.A．from west to east
B．from north to south
C．from east to west
D．haphazard