Tao Puts an Object Under a Cover and Her Baby

Multiple Choice

Tao puts an object under a cover and her baby delightedly finds it a few times. Then she puts it under another cover and her baby looks in the first hiding place. This is:

A) a sign of a psychological problem.
B) normal when babies are under 3 months old; it's called the A-not-B error.
C) normal as babies approach age 1; it's called the A-not-B error.
D) normal until children are 3 years old; it's called object permanence.