按搜索引擎的内容分，搜索引擎可以分为目录式搜索引擎和索引式搜索引擎。
参考答案：错
The place __ the bridge is supposed to be built should be ____the cross-river traffic is the heaviest .
A．which; where
B．at which ;which
C．at which ;where
D．which ;in which
一个环形，外圆的半径是5dm，内圆的半径是3dm，这个环形的面积是______dm2．
