The old scientist took up walking as a regular form of exercise, which, of course, is of great        to his heath.

A．benefit

B．favor

C．sense

D．relationship

饲料级硫酸锰中砷的含量应为（）

A、≤0.0005%

B、≤0.0006%

C、≤0.0007%

D、≤0.0008%