选A ,因为是many ,how many ,however many, 不管他们会遇到多么多的困难，他们都会继续努力，坚持下去知道克服这些困难。
A,猛一看A和B都差不多，但是词性有区别,However是副词，修饰形容词many
however many difficulties=no matter how many difficulties
Whatever 是形容词，不能修饰many
选A，how many 问“多少”，后街可数名词difficulties
选A。不管遇到多少困难～
A
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