A:No,I never have seen anything like that before
B:No,never I have seen anything like that before
C:No,never have 1 seen anything like that before
D:No,I have seen anything like that before never
如图,正六边形ABCDEF内接于⊙O.若直线PA与⊙O相切于点A,则∠PAB=____ .
①骨骼肌收缩 ②肌肉附着的骨收到牵拉产生动作 ③骨骼肌接受神经传来的兴奋