答案：B

题目分析：句意：你有没有发现许多年前恐龙灭绝的确切原因？died down （火）渐熄，  died out 灭绝，        died from 死于（外因，指伤病，饥饿等） died of 死于（内因，指疾病、过度悲伤等) People are dying of cancer more and more often. 死于癌症的人越来越多了。故选B。

考点: 考查动词短语辨析的用法。