Ma Yao wants to use shaping to teach her dog, Shardu, to play dead. In order to play dead, Shardu must lay on the floor with his feet up in the air for at least 5 s following the command, "play dead". Shardu can already sit, lay down, and roll over on command. However, he will only sit and lay down for a couple of seconds. What factors should Ma Yao consider when selecting the initial behavior to reinforce and why? What is the best behavior to initially reinforce?