B。

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。