用纯种的高茎豌豆和矮茎豌豆作杂交实验时，需要                                                       

完形填空
    This is an educational story．A teacher once told each of her students to bring an empty plastic bag and a large bag of 1 to school．If there was someone in their lives that they 2 to forgive，they chose a potato，  3  the name of that person and the date on it, and then put it  4 the plastic bag．Some of their bags were quite heavy．
    They were then told to5 these bags with them everywhere for one week，putting them beside their beds  6 and next to their desks in the classroom．
    As they carried the bags around with them，they 7 what a weight they were carrying，and how hard they needed to pay attention  8 ． Of course，the potatoes were rotten(腐烂的)soon，and smelled  9 ．
    This is the same as the price we pay for keeping our pain and unhappiness with us!So 10others is not only a gift for them，but also a very good gift for ourselves．

某班开展一次智力竞赛活动，共a，b，c三个问题，其中题a满分是20分，题b，c满分都是25分．每道题或者得满分，或者得0分．活动结果显示，全班同学每人至少答对一道题，有1名同学答对全部三道题，有15名同学答对其中两道题．答对题a与题b的人数之和为29，答对题a与题c的人数之和为25，答对题b与题c的人数之和为20．则该班同学中只答对一道题的人数是____；该班的平均成绩是____．

下列说法中，错误的是（  ）

在动摩擦因数μ=0.2的粗糙绝缘足够长的水平槽中，长为2L的绝缘轻质细杆两端各连接一个质量均为m的带电小球A和B，如图为俯视图（槽两侧光滑）．A球的电荷量为+2q，B球的电荷量为﹣3q（均可视为质点，也不考虑两者间相互作用的库仑力）．现让A球处于如图所示的有界匀强电场区域MPQN内，已知虚线MP恰位于细杆的中垂线，MP和NQ的距离为3L，匀强电场的场强大小为E= $\frac{1.2mg}{q}$ ，方向水平向右．释放带电系统，让A、B从静止开始运动（忽略小球运动中所产生的磁场造成的影响，重力加速度为g）．求：