参考答案

正确答案：B
解析：本题为细节题。解题的关键是第三段第二句话： “The growing intensity and complexity of travel patterns means controlling．health problems is becoming even more difficult．”这句话说明“控制健康方面的问题越来越难”主要有两个原因，原因之一是选项B．the way people travel today has become more complex，该选项与文章中“complexity of travel patterns”意思一致。选项A．individual health problem is ignored by both the government and the tourists在原文中并没有提到；选项C和选项D中所给出的原因与常识知识相关，但是在原文中都没有提到。