Alan makes hand$-$tooled leather items such as purses, luggage tags, and coasters for sale. Alan buys his leather from Sherman Oak, a company that deals in bulk leather, dyes, finishes, edge coats, clear coats, and cement. This is an example of

a$.$ consumer marketing

b$.$ social marketing

c$.$ internet marketing

d$.$ business to business $(B-$to$-$B$)$ marketing