Benjamin and Gustavo see a cluster of bushes on the playground. They decide they will build a fort out of it. They decide together that Benjamin will find something to make a door to close a gap, and Gustavo will make a bridge over an imaginary river. When they are done, they will take turns guarding the door and driving across the bridge. What type of play are they engaging in ?

a. cooperative
b. parallel
c. associative
d. onlooker