A:向沸水中滴加FeCl3溶液制备Fe(OH)3胶体:Fe3++3H2OFe(OH)3↓+3H+
B:向小苏打溶液中加入过量的石灰水:Ca2++OH-+HCO3-=CaCO3↓+H2O
C:向硫酸氢钠溶液中加入氢氧化钡使溶液呈中性:Ba2++OH-+H++SO42-= BaSO4↓+H2O
D:用食醋除去水壶中的水垢:CaCO3+2H+=Ca2++CO2↑+H2O
--- Has he returned the book to our library?
--- No. He has _______ it for several months.