Answer:
If Angela decides to reduce her consumption of dark chocolate, then the quantity that she demands of dark chocolate will be reduced, shifting her dark chocolate curve to the left.
Since Angela decided to increase her consumption of white chocolate, then the quantity that she demands of white chocolate will be increased, shifting her white chocolate curve to the right.
Since Angela's demand curve for dark chocolate shifts to the left, and only Vanessa and her consumed dark chocolate, then the total demand curve for dark chocolate will shift to the left.
