以下经济变量是存量还是流量？ a．蓝领工人的收入。 b．同一共人的财富。 c．对世界其他国家的净要素支付。 d．一国经济中所有房屋的价值。 Are the following economic variables stocks or flows？ a．The income of a blue-collar worker． b．The wealth of that same worker． c．The net factor payments to the rest of the world． d．The value of all the houses in the economy．