Helping animals in danger, the ________ is to stop people ________ them.A.difference; to killB.different; killingC.difficult; to killD.difficulty; killing
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D解析点拨：此题考查名词作主语和stop sb. (from) doing sth. 的用法。题意是“帮助濒危动物，难题就是阻止人们捕杀他们。”选项B和C中different和difficult是形容词，不能作主语；选项A中difference不符合题意，而且to kill也不符合stop... doing的结构；只有选项D中difficulty是名词作主语，killing也符合stop... doing的用法。