A 6-Month-Old Infant Watches as His Sister Hides Behind a Door

A 6-month-old infant watches as his sister hides behind a door. While the infant is turned away from his sister for a moment, the sister moves from behind the door and hides under a bed. Suddenly, the wind pushes the partially open door even further open, until it touches the wall. The 6-month-old is MOST likely to:

A) stare at the fully open door, expecting his sister to appear from behind it.
B) stare at the fully open door, surprised that the door was able to fully open with his sister behind it.
C) turn away from the door, demonstrating no surprise that the door was able to fully open with his sister behind it.
D) turn to his mother and ask where his sister went.