题干

电磁轨道炮工作原理如图所示.待发射弹体可在两平行轨道之间自由移动,并与轨道保持良好接触.电流I从一条轨道流入,通过导电弹体后从另一条轨道流回.轨道电流可形成在弹体处垂直于轨道面得磁场(可视为匀强磁场),磁感应强度的大小与I成正比.通电的弹体在轨道上受到安培力的作用而高速射出.现欲使弹体的出射速度增加至原来的2倍,理论上可采用的方法是(   )

A:只将轨道长度L变为原来的2倍

B:只将电流I增加至原来的2倍

C:只将弹体质量减至原来的一半

D:将弹体质量减至原来的一半,轨道长度L变为原来的2倍,其它量不变

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B,D

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请仔细阅读1—5小题,从所给的六个选项(A、B、C、D、E、F)中,选出符合各小题要求的最佳选项。选项中有一项是多余选项。首先,请阅读以下某时尚杂志提供的几条护发建议:阅读下面几种的情况介绍,并为他们匹配合适的建议。

    It's very important to keep your hair healthy. Here are five ways for you.

    ____Good fats are also called necessary fatty acids which the body can't produce naturally. Eating these types of fats will help improve healthy skins, nails, teeth, and hair. They can be found in the form of oil, or in food such as seeds, nuts.

    ____Hair is nothing more than dead protein(蛋白质).But that doesn't mean that it is not important. A lack of it can cause slow growth and thinning, where on the contrary, getting enough amounts can help growth. Good choices to include in the diet would be eggs, meats, tofu, fish and beans.

    ____There are a handful of vitamins that have been known to be effective for healthy hair. Some of them include vitamins A, B, C and E, and iron. Other than in extra form, these nutrients can also be found naturally in fruits, peppers, low-fat dairy products, green vegetables and whole grains. A large salad can be made with the inclusion of a lot of these foods.

    ____Getting enough exercises can help improve hair growth. Aim to get at least 20-30 minutes of exercise every day. It can be walking, jogging, weight training, and biking.

    ____Right after you brush your teeth, brush your hair as well. Brushing hair fifty hundred strokes before bed can help remove pollutants and dirt while also stimulating circulation. Also, natural oils are released which will add more sheen(光泽) and oxygen to your hair.

A. Take your vitamins.

B. Remember to brush before bed

C. Wash your hair now and then.

D. Get your protein.

E. Exercises are very important.

F. Eat good fats.