It is in their ability to speak humans from animals.
A. which; differ
B. that; different
C. that;differ
D. which;different
网友回答
A. which; differ
B. that; different
(答案→)C. that;differ
D. which;different
解析：
第一空考查的是强调句，第二空考查动词differ from“于…不同”，句意是：人类和动物的不同在于他们说话的能力。选C。