Margaret,who Is 55 Years Old,needs to Get Three Projects Done

Margaret,who is 55 years old,needs to get three projects done at work by the end of the week.When she was younger,she would have worked on all three projects more or less at the same time.Now that she is older,however,Margaret needs to work on one project at a time.Margaret's current inability to multitask is due to changes in her:

A) vision.
B) flexibility.
C) brain.
D) expertise.