磁头驱动机构由（）组成。A.电磁线圈电机 B.防震动装置 C.控制与接口电路 D.磁

声源振动时发出声音的频率即为每秒钟发出声波的数目，声源每秒钟发出的波数不同，发出声音的频率就不同．对观察者而言，人耳听到的声音频率即为每秒钟接收到的声波数目，例如我们听到频率为50Hz的声音，每秒钟接收到的声波数即为50．当声源靠近或远离观察者时，人耳听到的频率不再等于声源发出的频率，这种现象叫多普勒效应．

（1）我们在生活中经常有这样的经验：高速驶来的火车“呼啸而来，扬长而去”．试定性分析当火车靠近我们时，我们听到的鸣笛声与火车发出的声音频率相比有何不同？

（2）观察者静止在路旁，一列动车以速度v向观察者驶来，动车发出的鸣笛声频率为f₀，观察者听到的声音频率为f，声音的速度为u，试推导f与f₀的关系．

（3）利用多普勒效应可以测定动车的速度．观察者坐在速度为80km/h的普通列车上，从对面开来一列动车，迎面时听到动车鸣笛声的频率为f₁，动车向后奔驰而去时听到的频率为f₂，若f₁：f₂=2：1，声音的速度为1200km/h，求动车的速度大小．

下列君主中大力扶持佛教的是

A．北魏太武帝

B．梁武帝

C．唐武宗

D．北周武帝

霍乱患者解除隔离的标准是

A．临床症状消失4天，粪便隔日培养1次，连续2次阴性

B．临床症状消失6天，粪便隔日培养1次，连续3次阴性

C．临床症状消失7天，粪便隔日培养1次，连续3次阴性

D．临床症状消失8天，粪便隔日培养1次，连续4次阴性

E．临床症状消失10天，粪便培养1次阴性

Left-Handers’ Day

International Left-Handers’ Day is August the thirteenth. The International Left-Handers Day, Web site says the Left-Handers Club started the holiday in 1992. It wanted left-handers around the world to celebrate. And it wanted to bring attention to the everyday problems of people who use their left hands.

One of these problems is difficulty using equipment and tools, like scissors. In general, most tools and equipment are made for people who are right-handed. The Left-Handers Club tries to educate designers and producers to consider the safety of left-handed people when producing their products.

Another problem is that many people have considered it bad to be left-handed. Some teachers and parents have tried to force children who used their left hands to use their right ones instead.

Scientists do not really know why some people are left-handed. They have believed the reason is genetic.They say the gene（基因） increases the chance of being left-handed. It appears to play an important part in deciding which part of the brain controls different activities. In right-handed people, the left side of the brain usually controls speech and language. The right side controls feelings. However, the opposite is often true in left-handed people.

Scientists believe the gene is responsible for this. The gene showed a link with left-handedness in nine to twelve percent of the population.

About ten percent of people around the world are left-handed.

小题1:International Left-handers’ Day aims to（目的是） bring attention to_________.

A．the right-handers

B．the everyday problems

C．the people who have some difficulty using equipment and tools

D．the everyday problems of people who use their left hands小题2:How many problems of people who use their left hands are mentioned?

A．One.

B．Two.

C．Three.

D．Four.小题3:As to why some people are left-handed, scientists believe the reason is________.

A．habits

B．genetics

C．parents

D．birthplaces小题4:______of people around the world are left-handers.

A．About 9%

B．About 12%

C．About 9.12%

D．About 10%

炉膛、烟道吹灰汽源？