题目

My parents operated a small restaurant in Seattle．It was open twenty-four hours a day, six days a week．And my first real job，when I was six years old，was  1  the diners’ shoes．My father had done it when he was young，so he taught me  2  to do it efficiently，telling me to  3  to reshine the shoes if the customer wasn’t  4  ．        Working in the restaurant was a cause of great   5   because I was also working for the good of the family．But my father  6  that I had to meet certain standards to be part of the team．I  7  to be punctual，hard-working如图，粗糙的水平地面上有一斜劈，斜劈上一物块正在沿斜面以速度v0匀速下滑，斜劈保持静止，则地面对斜劈的摩擦力 [     ]A.等于零 B.不为零，方向向右 C.不为零，方向向左 D.不为零，v0较大时方向向左，v0较小时方向向右