城市道路用地面积与城市用地面积之比，称为（）
A.非直线系数
B.干道网密度
C.道路占有率
D.道路面积密度
参考答案：D
Light-to-moderate smokers were ________ to die of sudden heart problems than those who had never smoked.
A．as twice likely
B．twice more possible
C．twice more likely
D．as twice possible
什么是观察？
