Baby caitlen is about to turn one year old. she loves to play peek-a-boo and acts so surprised each time a hidden face appears behind her dad's hands. why is she surprised?

Caitlen doesn't that each times her dad hides he still exists.

Explanation:

Baby Caitlen is still one year old this means that she is still learning that things still remain when they disappear and come back in sight. The psychologist Jean Piaget called this principle "object permanence". As the baby grows older this effect seems to wean off. The baby is learning that things do not disappear but are merely out of sight.

a boiling water reactor does not boil the water. a pressurized water reactor boils the water faster. a pressurized water reactor is heated directly by the core. a pressurized water reactor does not use fission.

a:

by the combination of an unlimited number of elements