It is in their ability to speak humans from animals.A. which；differB. that；differentC. w

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A. which；differ
B. that；different
C. which；different
(答案→)D. that；differ
解析：考查强调句型和固定词组。强调句型：it is/was被强调成分that/who其它成分。强调句型最大的特点是把it is/was…that/who…去掉，句中仍然是对的。本题第一空正是强调句型，强调的是in their ability to speak，第二空是固定词组differ from…与…不同。句意;正是在说话的能力父母，人们区别与动物。故D正确。