L$.$L$.$ Bean allows customers to return merchandise at any time if not completely satisfied: $"$We make pieces that last, and if they don't, we want to know about it$.$ So if it's not working or fitting or standing up to its task, we'll take it back. L$.$L$.$ himself always said that he didn't consider a sale complete 'until goods are worn out and the customer still satisfied."$\text{'}$ This philosophy is consistent with $q,$

a$.$ vertical integration

b$.$ differentiation strategy

c$.$ strategic business unit

d$.$ corporate portfolio matrix