点O是平行四边形ABCD对角线的交点，若平行四边行ABCD的面积为8cm2，则△AOB的面积为（ ）。
2
Finally he reached a lonely island ________ was completely cut off from the outside world.
A．when
B．where
C．which
D．whom
树是结点的集合，它的根结点数目是
A．有且只有1
B．1或多于1
C．0或1
D．至少2
