Research Using the Shadowing Technique to Study Attention Has Led

Research using the shadowing technique to study attention has led psychologists to conclude that:

A) people cannot attend to more than one thing at a time and are unable to rapidly shift attention between two stimuli.
B) with practice, people can perfectly attend to three or more messages simultaneously.
C) with practice, people can perfectly attend to two messages simultaneously.
D) people cannot attend to more than one thing at a time, but are able to rapidly shift attention between two stimuli.