The Following Graph Shows Three Relationships as a Function of Plasma

The following graph shows three relationships as a function of plasma [glucose]. At plasma [glucose] less than 200 $\mathrm{mg} / \mathrm{dL}$ , curves $\mathrm{X}$ and $\mathrm{Z}$ are superimposed on each other because

A) the reabsorption and excretion of glucose are equal
B) all of the filtered glucose is reabsorbed
C) glucose reabsorption is saturated
D) the renal threshold for glucose has been exceeded
E) $\mathrm{Na}^{+}$ -glucose cotransport has been inhibited
F) all of the filtered glucose is excreted