B

    We are learning English, but how can we learn English well? A student can know a lot about English, but maybe he can't speak English.

If you want to know how to swim, you must get into the river. And if you want to be a football player, you must play football.

    So you can learn English only by using it. You must listen to your teacher in class. You must read English every day. You must speak English to your classmates and you must also write something in English sometimes. Then one day, you may find your English very good.

Ca²⁺____； $\stackrel{-2}{S}$ ____；N₂____．

选出下列句子内容表述不正确的一项（  ）。