Brenda Has Trouble Remembering Her New Five-Digit ZIP Plus Four-Digit

Brenda has trouble remembering her new five-digit ZIP plus four-digit address code. What is the most likely explanation for the difficulty Brenda is having?

A) Nine digits are at or above the upper limit of most people's short-term memory capacity.
B) Nine digits are at or above the upper limit of most people's iconic memory capacity.
C) The extra four digits cannot be organized into easily remembered chunks.
D) Brenda evidently has an impaired implicit memory.