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