What Do Researchers Mean When They Describe Play as a "Serious

What do researchers mean when they describe play as a "serious business"?

A) Play is much more demanding for children than it appears to an adult observer.
B) Unsupervised play has serious aversive outcomes for intellectual and social development, unless appropriate outcomes are modelled by adults.
C) Play episodes are among the most complex interactions in which preschool children engage.
D) All children's play should be structured by adults so it involves educational opportunity.