Mr. Brown has made a of three books for us to read during the school holiday.
A:map
B:rule
C:list
D:menu
已知P,Q是圆心在坐标原点O的单位圆上的两点,分别位于第一象限和第四象限,且P点的纵坐标为45,Q点的横坐标为513,则cos∠POQ=____
下列各类生物细胞中,具有成形细胞核的是( )