某同学用单色光进行双缝干涉实验，在屏上观察到如图甲所示的条纹，仅改变一个实验条件后，观察到的条纹如图乙所示．他改变的实验条件可能是(　　)

      On weekdays, David gets up at 6:30.He has breakfast at seven and then he goes to school by bus. He gets there at 7:15. He doesn't like to be late. Classes begin at 8:00. He has lunch at twelve at school. He goes home at 5:30 in the afternoon. On weekends, he gets up at about 8:00 and makes breakfast for his family. His parents like him very much.

利用弹簧测力计测量一块秒表的重力时，使测力计内弹簧伸长的力是（　　）

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