Scenario 12-10 Walter Builds Birdhouses.He Spends $5 on the Materials for Each

Scenario 12-10
Walter builds birdhouses.He spends $5 on the materials for each birdhouse.He can build one in 30 minutes.He is semi-retired but earns $8 per hour at the local hardware store.He can sell a birdhouse for $20 each.
-Refer to Scenario 12-10.An accountant would calculate the total cost for one birdhouse to be

A) $5.
B) $8.
C) $9.
D) $13.