Fill in the blanks appropriately:
Organ: Excretory wastes
$(a)$ Kidneys: .......
$(b)$ Lungs: ...........
$(c)$ Liver: ...........
$(d)$ Skin: ...........

(N/A) Kidneys excrete urine,which contains water,urea,and other salts.
$(b)$ Lungs excrete $CO_{2}$ (carbon dioxide) and water vapor during respiration.
$(c)$ Liver excretes bile pigments (bilirubin and biliverdin) and cholesterol.
$(d)$ Skin excretes sweat,which contains water,small amounts of urea,and salts.