Schwan's Sales Enterprises of Marshall, Minnesota, markets a full line of frozen foods in $49$ states and parts of Canada using door$-$to$-$door salespeople who sell from refrigerated trucks. This particular method of distribution is called a $\_\_\_\_\_\_\_\_\_\_\_\_.$

a$.$

direct channel.

b$.$

indirect channel.

c$.$

facilitated channel.

d$.$

customer$-$service channel.

e$.$

truck jobber channel