Four-Month-Old Emile Watches as a Researcher Rolls a Ball Down

Four-month-old Emile watches as a researcher rolls a ball down a plank, through a colorful tunnel. Although he can smoothly track the progress of the ball's movement down the track, he doesn't look to the other side of the tunnel until the ball pops out. On the other hand, 7-month-old Bella anticipates its appearance after entering the tunnel and shifts her eyes to the end of the tunnel before the ball appears. Compared to Emile, Bella shows

A) motion parallax.
B) depth perception.
C) attractiveness effect.
D) anticipatory eye movements.
E) visual saccades.