Write the next sentence in this paragraph:

How to build a milk carton birdhouse
Wash the carton well.
Milk left inside the carton can mix with birdseed and go moldy. Ensure that the container is properly dry before proceeding.

Possible answers:
(A). Add 1 inch (1.3 cm) thick potting mix to the container. Mix the potting mix with your hands or a spoon..
(B). Use a sieve to remove soap scum. Use a heavy object to gently dislodge the scum..
(C). Cut out a rectangular piece of ground wood. The lumber should be at least three feet (eight inches) wide and 12 inches deep enough to accommodate the holding posts..
(D). Paint the entire milk carton with primer. When it dries, draw a design of your choosing on the milk carton with the pencil..
(D).