问题如下：
The production relationship between the number of machine hours and total product for a company is presented below.
Diminishing marginal returns first occur beyond machine hour:
选项：
A.3.
B.4.
C.5.
解释：
A is correct.
Diminishing marginal returns occur when the marginal product of a resource decreases as additional units of that input are employed. Marginal product, which is the additional output resulting from using one more unit of input, is presented below.
The marginal product of the third machine hour is 6 and declines thereafter. Consequently, diminishing marginal returns are first evident beyond three machine hours.
考点：边际效用递减
我们首先计算不同产量下的边际产量，如下表所示。
可以看到产量由3增加到4的时候，边际产量由6下降到5，所以产量3的边际效用开始出现递减。
不是特别理解，能仔细解答下么？ 为什么产量都想赢减少了。
