题目

    For the first time, Zhang Shengyu, a student from Laiyang Middle School in Shandong, didn’t ask her mother to buy her a new schoolbag or any new pens for the beginning of the school term. “My old schoolbag can still be used, so there is no need to buy a new one,” Zhang said, “I’ve decided to be thrifty from now on.”    As the center of the family’s attention, many of the one-child generation (一代人) do not know about the hardships of life. They sometimes have no idea of the value of things. Many students would throw away pens and erasers that haven’t been used up 下列各组气体等物质的量混合后，得到的混合气体的平均式量可能为30的是（    ） A．CO2和CO　　 B．SO2和CO2    C．N2和CO　　 D．N2和O2