题目

Seeing the happy _____ of children playing in the park, I’m full of joy and confidence in the future of our country. A. sight        B. scene        C. view         D. sign 答案：B20．质量分别是m1和m2的两个木块用轻弹簧相连，放在水平地面上，如图所示，用细线拴住m1，并用力将它缓慢竖直向上提起，当木块m2刚要离开地面时，细线突然断裂，则此时木块m1的加速度为（　　）A．0B．gC．$\frac{{（{m_1}+{m_2}）g}}{m_1}$D．$\frac{{{m_2}g}}{m_1}$