题目

阅读下列短文，从每题所给的四个选项（A、B、C和D）中选出最佳选项。One afternoon, after finishing shopping in a supermarket, my family and I went to the checkout. I suddenly thought we didn't need any of the junk, and we abandoned all of those, saving $300.That got me thinking about all our pointless expenses in life. With a promise that we'd stop if it was killing us, I convinced the family to take the leap into frugality (节俭). The rules were that we would buy nothing for 30 days except absolutely essentials.Our adventure began with a great start. By 9 a.m., my （8分） 现要测定木块与长木板之间的动摩擦因数，给定的器材如下：一倾角可以调节的长木板（如图）、木块、计时器一个、米尺．填入适当的公式或文字，完善以下实验步骤：①让木块从斜面上方一固定点D从静止开始下滑到 斜面底端A处，记下所用的时间t． ②用米尺测量D与A之间的距离s，则木块的加速度a＝　　　　 ．③用米尺测量长木板顶端B相对于水平桌面CA的高度h和长木板的总长度l．设木块所受重力为mg，木块与长木板之间的动摩擦因数为，则木块所受的合外力F ＝_____ ．④根据牛顿第二定律，可求得动摩擦因数的表达式＝__________________________ ，代入测量值计算求出的值． ⑤改变____________________________________________，重复上述测量和计算．⑥求出的平均值．