题目

Only when class began _______that he had left his book at home. A. will he realizeB. he did realize C. did he realizeD. should he realize 答案：C 解析:  only+状语放于句首，应构成倒装句，主语前要加助动词，由时态看为一般过去时。一根轻质弹簧原长为20cm，其上端固定，下端竖直悬挂一个重为10N的静止物体，此时弹簧的长度为30cm，如图所示，则弹簧的劲度系数为______N/m．（已知弹簧的形变量在弹性限度范围内）