若单项式-$\frac{2x^{2}y}{3}$的系数是m，次数是n，则mn的值为    （    ）

下列有关实验的操作或叙述正确的是（　　）

在“探究平抛运动的运动规律”的实验中可以描绘出小球平抛运动的轨迹，实验简要步骤如下：

We have six lessons today. At 8:00 a. m. we have Chinese. At 8:50 a.m. we have Maths. At 10:00 a. m. we have Science. At 10:50 a.m. we have Music. From 11:30a.m. to 1:30 p.m. we have a lunch break. In the afternoon, we have English and Art.