如图所示，在互相垂直的匀强电场和匀强磁场中，电荷量为q的液滴在竖直面内做半径为R的匀速圆周运动．已知电场强度为E，磁感应强度为B，则液滴的质量和环绕速度分别为
A．
，
B．
，
C．B 
，
D．，
D
题目分析:液滴在重力、电场力和洛仑兹力下做匀速圆周运动，可知，液滴受到的重力和电场力是一对平衡力，重力竖直向下，所以电场力竖直向上，与电场方向相同，mg=qE，得m=
；液滴在洛伦兹力的作用下做匀速圆周运动的半径为R=
，得v=
，故D正确。
| 阅读理解。 | |
| |
| 1. On Sunday the zoo opens at ______. | |
[ ] | |
| A．10:00 in the morning B．8:00 in the morning C．6:00 in the afternoon D．8:00 in the afternoon | |
| 2. If you are 13, how much do you pay to go into the zoo? | |
[ ] | |
| A．3 dollars. B．6 dollars. C．8 dollars. D．Nothing. | |
| 3. Which of the following statements is true? | |
[ ] | |
| A．The zoo opens longer on Saturday and Sunday than on other days. B．Children under 8 can go into the zoo at any time without paying anything. C．Adults have to pay 17 dollars to go into the zoo. D．You can't take a train to the zoo on weekdays. | |
| 4. If Lucy is 14 and her brother is 7, how much do they pay for the train ride? | |
[ ] | |
| A．2 dollars. B．4 dollars. C．6 dollars. D．8 dollars. |