Listening attentively in class and reviewing in time _______ a sure way to improve your learning, and this is especially true _______ it comes to standard tests.

A．is, when

B．are, when

C．is, while

D．are, as

双硫腙比色法测定铅时，为了防止铁、铜、锌等离子的干扰，应加入（）。

A、柠檬酸铵

B、氰化钾

C、盐酸羟胺

D、以上都是