题目

Yawning sends out certain messages -either “Oh ，this movie is so boring” or “I probably need to get some sleep”. But did you know that a yawn can also help your brain to cool down when it is overheated? A new study, led by a research team at Princeton University, has indicated that yawning could be the brain’s natural way of regulating temperature. People yawn more often when the temperature outdoors is lower than their body temperature but are less likely to yawn when it is hotter outdoors, according to Sciencedaily. com. The research team did an experiment on 160 people, 圆锥的侧面展开图是直径为a的半圆面，那么此圆锥的轴截面是       （    ）        A．等边三角形                    B．等腰直角三角形        C．顶角为30°的等腰三角形       D．其他等腰三角形