Please choose the correct ending to complete the given sentence: Pets and Animals: [header] How to build a milk carton birdhouse [title] Wash the carton well. [step] Milk left inside the carton can mix with birdseed and go moldy. Ensure that the container is properly dry before proceeding.

Ending1: [title] Add 1 ½ inch (1.3 cm) thick potting mix to the container. [step] Mix the potting mix with your hands or a spoon. Ending2: [title] Use a sieve to remove soap scum. [step] Use a heavy object to gently dislodge the scum. Ending3: [title] Cut out a rectangular piece of ground wood. [step] The lumber should be at least three feet (eight inches) wide and 12 inches deep enough to accommodate the holding posts. Ending4: [title] Paint the entire milk carton with primer. [step] When it dries, draw a design of your choosing on the milk carton with the pencil.

Answer format: ending1/ending2/ending3/ending4