用托盘天平称量物体的质量时，将被称物体放在了右盘，而砝码放在了左盘，若天平平衡时，左盘放有100克和20克的砝码各1个，游码所对的刻度值是4克，则物体的质量为 ( ）
A．124克
B．122克
C．118
D．116克
答案：D
题目分析：根据天平的构造可知，读数时应为左盘物体的质量=右盘砝码的质量+游码所对的刻度值，本题将物体、砝码互换可得结果．根据天平的构造，100g+20g=物体的质量+4g，所以物体质量为：120g-4g=116g；故选D．
点评：天平在使用时应左托盘放称量物，右托盘放砝码，放反后可将砝码看作有固定质量的物体，而物体可看作未知质量的物体即可．
| 完形填空。 | |||
| Ann is__1__English girl. She is fourteen. She is a__2__student. Her teacher is Miss White. Ann helps her teacher to put__3__books and ruler on the desk. They have a new__4__. In it you can __5__40 desks and chairs. A blackboard, a clock and a map are__6__the wall. There are some pens on the desks. The red__7__is Ann’s, others are Kate’s. Kate is Ann’s __8__friend. Kate isn’t__9__here today. She is ill. Look at the clock. It’s about four o’clock. It’s time__10__to play games. | |||
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