问题 选择题
如图所示,在x轴上方存在着垂直于纸面向里、磁感应强度为B的匀强磁场,一个不计重力的带电粒子从坐标原点O处以速度v进入磁场,粒子进人磁场时的速度方向垂直于磁场且与x轴正方向成120°角,若粒子穿过y轴正半轴后在磁场中到x轴的最大距离为a,则该粒子的比荷和所带电荷的正负是(  )

A.,正电荷

B.,正电荷

C.,负电荷

D.,负电荷

答案

答案:C

阅读理解

阅读理解。

      Do you love holidays but hate the increase weight that follows? You are not alone.

      Holidays are happy days with pleasure and delicious foods. Many people, however, are worried about the

weight that comes along with these delicious foods.

      With proper planning, though, it is possible to control your weight. The idea is to enjoy the holidays but

not to eat too much. You don't have to turn away from the foods that you enjoy. The following suggestions

may be of some help to you.

      Do not miss meals. Before yon leave home for a feast (宴会), have a small, low-fat snack (小吃). This

may help to keep you from getting too excited before delicious foods. Begin with clear soup and fruit or

vegetables. A large glass of water before you eat may help you feel full. Use a small plate; a large plate will

encourage you to have more than enough. 

      Better not have high-fat foods. Dishes that look oily or creamy have much fat in them.

      Choose lean meat (瘦肉). Fill your plate with salad and green vegetables.

      If you have a sweet tooth, try mints (薄荷) and fruits. They don't have fat content as cream and

chocolate.

      Don't let exercise take a break during the holidays. A 20-minute walk after a meal can help burn off

excess (过多的) calories (卡路里).

1. Holidays are happy days with pleasure but they may _____. [ ]

A. bring you much trouble in your life

B. bring weight problems

C. make you worried about your foods

D. make you hate delicious foods

2. In order to really enjoy your holidays without putting on weight, you'd better _____. [ ]

A. not eat the food in high fat

B. drink much water and have vegetables only

C. not accept invitations to feasts

D. turn away from delicious foods

3. According to the passage, _____ is a necessary part to stop you from putting on weight.[ ]

A. vegetables

B. water

C. calories of energy

D. physical exercise

4. Excess calories can be found in your body in the form of _____. [ ]

A. energy

B. fat

C. food

D. something invisible (看不见)

单项选择题