$.$ Gleeson Timber Mill produces three timber products in a common$/$joint production process. The sawdust that is produced incidentally during the production process is sold to local farmers. The revenues generated from the sale of this by$-$product should be:

a$)$ Deducted from the cost of the joint process

b$)$ Treated as a variable cost

c$)$ Included as part of sales revenue

d$)$ Treated as petty cash