Which of the following is correct regarding a companys make or buy analysis?

a) A disadvantage of vertical integration is that by pooling demand for parts from a number of companies, a supplier may be able to enjoy economies of scale that result in higher quality and lower cost than if every company makes its own parts.

b) When a company is involved in one activity in the entire value chain, it is vertically integrated.

c) A vertically integrated company is more dependent on its suppliers than one that is not vertically integrated.

d) The variable selling cost of the item is relevant in the decision to make or buy an item.