观察图形的变化规律，写出第n个小房子用了____块石子．

    It's Sunday morning. The students of Class 3 are giving their classroom a good cleaning. Miss Huang, their teacher, is working with them. The children are busy. Some are carrying water, some are cleaning the windows, others are sweeping the floor. Zhang Hua is putting up a map on the wall. It is a map of China. Wang Fei and Wei Qing are mending some broken chairs. The children are listening to the radio while they are working. The classroom looks nice and bright after the cleaning. The children are very happy. They go home for lunch at noon.

在“验证机械能守恒”的实验中，有下述A至F六个步骤：

A．将打点计时器竖直固定在铁架台上

B．接通电源，再松开纸带，让重物自由下落

C．取下纸带，更换新纸带，重新做实验

D．将重物固定在纸带的一端，让纸带穿过打点计时器，用手提着纸带，让重物靠近打点计时器

E．选择一条纸带，用刻度尺测出物体下落的高度h₁、h₂、h₃、…h_n，计算出对应瞬时速度v₁、v₂、v₃…v_n

F．分别计算出 $\frac{1}{2}$ mv_n²和mgh_n在误差范围内是否相等．

下列句子中划线的词语使用不恰当的一项是（    ）