Why is the air quality tested every day? A．Just in case someone asks． B．Scientists like to take tests． C．Scientists are not very busy． D．To keep track of pollution levels.
A.Just B.Scientists C.Scientists D.To
【参考答案】
D 见第三、四段。科学家每天检测，空气中这种有害颗粒的密度。如果密度高于65毫克/立方米(科学家认为这个标准还应......