Mother ______ the baby gently on the bed and then bent down to pick up the toy _____ on the ground.

A．laid; lying

B．laid; lie

C．lay; lying

D．had laid; lay

3，5B.r2-PA.DA.T分光光度法测定水中银含量时，水样叮对于银含量20含倍的铬（III）对测定无干扰。