What Does the Nurse Recognize as an Example of Piaget's

What does the nurse recognize as an example of Piaget's concrete operational thinking?

A) A 2-year-old child says,"It's nighttime" when his room is darkened.
B) A 4-year-old child refers to the hospital as "my house."
C) A 5-year-old child coloring a picture of a puppy says,"This is my puppy."
D) A 7-year-old child says,"I am sick because I have germs in my chest."