Objective To evaluate in laboratory the stain prevention and removal efficacy of Whitening Dentifrice,containing the complex stain removal active components—sodium and potassium salts of pyrophosphate acid(6.

Underground Drainage can be divided into underground level and vertical drainage,waterlogged removal and regulating soaking,but the ground drainage only drains flood,can t drain waterlogged by the earthsurface accumulated water,it is finished by underground drainage.