Why Have Earlier Studies of Short-Term Memory Capacity Overestimated Its

Why have earlier studies of short-term memory capacity overestimated its capacity,according to Cowan

A)  The studies failed to control for prior exposure to familiar test items. 
B)  Researchers used university students with above-average memory abilities. 
C)  Researchers used nonsense syllables, which are unnatural stimuli. 
D)  The studies failed to control for participants' tendency to chunk information.