钙发挥生理作用的主要形式是
A.扩散性非离子钙
B.非扩散钙
C.扩散性离子钙
D.结合钙
E.复合钙
参考答案:C
The window was broken. Try to ________ who has broken it.[ ]
A. find
B. look
C. find out
D. look for
The best characterization of a firm that is operating on its long-run average cost curve is when it:()
A. experiences constant returns to scale.
B. produces a given output at the least possible cost.
C. chooses a plant size that minimizes the average fixed cost.