David his friend's invitation to the picnic in order to stay at home and study for the test.
A:expected
B:refused
C:received
若椭圆x2a2+y2b2=1(a>b>0)与双曲线3x2a2﹣3y2b2=1共焦点,则双曲线的渐近线方程为( )
关节活动起来非常灵活,与之有关的结构特点是关节结构里有( )
有位同学用如下图所示的装置来探究光的反射规律,竖直纸板可沿虚线折叠.