题目

After an absence of thirty years, I decided to visit my old school again．I had expected to find changes, but not a completely different building．As I walked up the school drive, I wondered for a moment if I had come to the right address．The dirty red brick houses had been swept away．In its place stood a bright, modern block．A huge expanse of glass extended (延展) across the face of the building, and in front, there was a well-kept lawn (草坪) where previously there had been “an untidy gravel (沙石) yard”. I was pleased to find that it was bread time and that the ch在验证力的平行四边形定则的实验中，橡皮绳的一端固定在木板上，用两个测力计把橡皮绳另一端拉到一个确定的点O。下面提供的后继操作步骤中错误的是      。A．实验中测力计必须与木板保持平行B．读数时视线要正对测力计的刻度C．实验中用两弹簧秤同时把橡皮绳的的另一端O点时，两弹簧秤之间的夹角不得大于90°D．实验时先用一把弹簧秤拉橡皮绳并使读数达到最大值，再用另一把弹簧秤拉橡皮绳，使它的另一端与O点重合。