When maternal blood volume increases, pregnant women receive supplemental iron to prevent anemia. This is because:

a. the mother's red blood cells do not carry as much oxygen as they did before the pregnancy.
b. there is a decrease in the number of the mother's red blood cells because they are now shared with the fetus.
c. although both red blood cells and plasma increase, there is slightly more plasma.
d. there is only an increase in plasma, not in red blood cells.

c