题目解析：
教授提到了山魈餺出牙齿的两种情况。一种是宽宽地咧开嘴做出紧张地打哈欠的动作，另一种是安静地露出牙齿的动作。其中，静静地露出牙齿的表情常常被视为威胁的意思。但实际上，它只是山魈的一种微笑。因此正确答案选 A。
【对应原文】
which is the exact opposite of when the mandrill silently bares its teeth, its canines and premolars. There is a common misconception that this signifies a threat of aggression. In fact, it's actually a sign of friendliness, submission, or peace...kind of a like a grin. Infants perform this display during and before play activities.
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