问题补充：
Human facial expressions differ from those of animals in the degree they can be controlled on purpose.
A. with which
B. to which
C. of which
D. for which
答案：
A. with which
(答案→)B. to which
C. of which
D. for which
解析：to which中的which指代前面提到的degree，由于表示“在……程度上”时，degree前面习惯上要搭配介词to，故要选B。又如：To what degree are you interested in Japanese? 你对日语的兴趣达到了什么程度? At that time politicians were not known to the degree that they are today. 那时政治家们并不像今天这样为人所知晓。题干的意思是：人类与动物面部表情的不同之处在于，人类可以在一定程度上有意识地控制其表情。
