Psychologists Walk and Gibson attempted to determine whether infants had developed the ability to perceive depth by

a. having them crawl over what appeared to be a sharp ledge.

b. measuring how long they looked at novel stimuli.

c. throwing soft foam balls to them and seeing how they would react.

d. showing them a simulation of birds flying at their faces.

e. seeing if they reached for a toy when it was held slightly beyond the reach of their arms.