阅读宋代至清代中国书院性质状况表：
上表呈现的变化说明（ ）
Restaurant
Feature(特征)
Sea House
It is ____but ____
Hometown Restaurant
It has ____
Friends Restaurant
It is ____ but has ____
如图△ABC的内接圆于⊙O，∠C=45°，AB=4，则⊙O的半径为（ ）
