下列说法正确的是
| A．SO2和NO2都是有用的化工原料，因此目前都是将汽车尾气中的氮氧化合物收集后制备硝酸以防止污染空气； |
| B．由于HClO具有漂白性，因此工业上常用Cl2的水溶液来漂白纸浆、毛、丝、草帽辫； |
| C．稀土元素在自然界中含量非常低，难以开采，但它们在冶金工业中有着重要的用途，被称为冶金工业的维生素； |
| D．金属钠通常被保存在煤油或石蜡油中，以隔绝空气； |
Encourage your children to try new things, but try not to ___ them too hard.
| A．draw | B．strike | C．rush | D．push |
The computer was used in teaching. As a result, not only__________, but students became more interested in the lessons. 
| A．saved was teachers’ energy | B．was teachers’ energy saved |
C．teachers’ energy was saved | D．was saved teachers’ energy |
The children all turned ____ the famous actress as she entered the classroom. 
| A．looked at | B．to look at |
C．to looking at | D．look at |
Edward, you play so well. But I ______ you played the piano. 
| A．didn’t know | B．hadn’t known |
C．don’t know | D．haven’t known |
She brought with her three friends, none of ______ I had ever met before. 
| A．them | B．who | C．whom | D．these |