【单选题】If the production possibilities curve is a straight line:
A.
opportunity costs increase as output of either commodity is expanded.
B.
resources are not equally substitutable between the production of the goods.
C.
opportunity costs are negative.
D.
as an increasing quantity of resources are shifted from one manufacturing process (good Y) to another (good X), there is no increase in opportunity cost.
【判断题】If we draw a straight line through the points in a scatter diagram and most of the points fall close to the line, we say that there is a positive linear relationship between the two variables.
【单选题】光滑曲面与竖直平面的交线是抛物线 , 如图 5 所示 , 抛物线的方程为 y = x 2 , 其下半部处在一个水平方向的匀强磁场中 , 磁场的上边界是 y = a 的直线 ( 图中的虚线所示 ), 一个小金属块从抛物线上 y = b ( b > a ) 处以速度 v 沿抛物线下滑 , 假设抛物线足够长 , 则金属块在曲面上滑动的过程中产生的焦耳热总量是 ( )