小玲每天骑自行车或步行上学，她上学的路程为2800米，骑自行车的平均速度是步行平均速度的4倍，骑自行车比步行上学早到30分钟．设小玲步行的平均速度为x米/分，根据题意，下面列出的方程正确的是（ ）

地图的类型多种多样，人们应根据需要选择常用地图，查找所需要的地理信息．读下列比例尺不同的三幅地图，回答问题．

B

    I'm at Billy's birthday party. There are a lot of friends in his house. Some stand beside the table and enjoy the snacks and drinks. There is some food on the table. I like the chocolate and the fruit very much. Billy sits in the sofa and chats with some friends there. He wears a red T-shirt and a pair of blue jeans. He looks so cool today. Wow! The cake is really big. There are some candles on it. Billy's friends begin to sing 'Happy Birthday' to Billy. Billy is very excited. He blows out the candles (吹蜡烛) and cuts the cake. Everyone has fun at the party.

在空中飞行的足球具有动能、重力势能，同时还具有 ____能，足球落地静止后，这三种能量不为零的是 ____能．