Diminishing returns set in after the truck is purchased.

The answer in the space provided is second. The diminishing returns set happens when there is an increase with the input variable and by this, it will likely cause the output to decrease as the marginal increase and in the same time, other inputs remains to be in constant.

answer; ///i believe the answer is (d) procedures to control the quailty of your product;

answer; (a) calling people who have not shown interest in the product and asking them to buy it;