题目

Saturday, June 16th In the morning, we went camping along a road in Green Natural Park. We were happy because it was a sunny day. We were expecting an enjoyable two－day holiday. On the way, we kept singing and making jokes.However, in the afternoon, when we finished our lunch at one oˈclock, it was dark and windy. Soon, it rained heavily. Unluckily, none of us brought an umbrella. We ran about but we could find no place to stay.Twenty minutes passed and it was still raining. There were hours to go before we reached the campsite(野营地). It was even worse that our small compass(指南针)如图（1），以梯形OABC的顶点O为原点，底边OA所在的直线为轴建立直角坐标系．梯形其它三个顶点坐标分别为：A（14，0），B（11，4），C（3，4），点E以每秒2个单位的速度从O点出发沿射线OA向A点运动，同时点F以每秒3个单位的速度，从O点出发沿折线OCB向B运动，设运动时间为t．（1）当t=4秒时，判断四边形COEB是什么样的四边形？（2）当t为何值时，四边形COEF是直角梯形？（3）在运动过程中，四边形COEF能否成为一个菱形？若能，请求出t的值；若不能，请简要说明理由，并改变E、F两点中任一个点的运动速度，使E、F运动到某时刻时，四边形COEF是菱形，并写出改变后的速度及t的值