Have you ever seen a horse with toes(脚趾)？Millions of years ago, horses had many toes. They had four toes on each front foot. They had three toes on each back foot. The horses were no bigger than the cats.These small horses lived in the forest. The toes helped the horses run on the soft, wet ground.
At first, it was very hot in the forest. But the weather changed. It became very cold. Many trees could not live in cold weather. The trees died and fell. Open fields took the place of forests. The sun made the ground dry and hard.
Horsed began to change, too. They began to get bigger. This took a long time. On the dry, hard land, horses needed only their middle toes for running. Their middle toes became hard. After a long time, horses had only one hard toe on each foot. Now we call this hard toe a hoof.
小题1:How many toes did a horse have millions of years ago?
A．14
B．12
C．16
D．13小题2:What does the underlined word “hoof” mean in Chinese?
A．尾巴
B．马鬃
C．腿
D．蹄子小题3:What was the weather like in the forest at first?
A．Very warm
B．Very hot
C．Very cold
D．Very cool小题4:Which is WRONG according to the passage?
A．The horses were much bigger than the cats millions of years ago.
B．The trees died and feel because the weather changed.
C．On the dry, hard land, horses needed only their middle toes for running.
D．The horses have fewer toes than before.小题5:The main idea of the passage is that_______________.
A．horses had three toes on each back foot.
B．horses lived in the hot forest
C．horses changed a lot over the years
D．horses didn’t like the cold weather
的带正电的粒子。在直线MN的右侧分布有范围足够大的匀强磁场，磁感应强度B=0.01
T，方向垂直纸面向里，MN与中线OO′垂直。两平行金属板间的电压U随时间变化的U—t图线如图乙所示。已知带电粒子的荷质比
，粒子的重力和粒子之间的作用力均可忽略不计，若
时刻粒子源放出的粒子恰能从平行金属板边缘离开电场（设在每个粒子通过电场区域的时间内，可以把板间的电场看作是恒定的）。求：
θ＝45°（2）
① 
②
③
④
θ＝45°
和
联立求出正确结果，参照上述评分标准给分。