When _____ help, one often says "Thank you" or "It's kind of you".[ ]

A. offering

B. to offer

C. to be offered

D. offered

离心泵闭式叶轮效率较高，应用最广，适用于输送（）。

A、污水

B、含沙的泥水

C、纯净的液体

D、含固体颗粒的液体