题目

根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。Is Fresh Air Really Good for You? We all grew up hearing people tell us to go out and get some fresh air. According to recent studies, the answer is a big YES, if the air quality in your camping area is good. If the air you're breathing is clean-which it would be if you're away from the smog of cities-then the air is filled with life-giving, energizing oxygen. If you exercise out of doors, your body will learn to breathe more deeply, allowing even more oxygen to get to y将标有“6V 3W”字样和标有“9V 6W”的两盏灯串联在某电源两端，只有一盏灯正常发光，另一盏灯发光较暗，则哪一盏正常，电源的电压是多少？