My grandfather is over 90 years old,_____he's in excellent condition.
A:so
B:if
C:but
D:or
产生白化病的根本原因是( )
若菱形的两条对角线长分别是6和8,则此菱形的周长是____ ,面积是____
设两圆C1、C2都和两坐标轴相切,且都过点(4,1),则两圆心的距离|C1C2|=____