阅读理解。
After the coming of Europeans to the Americas, Indians were forced to change their
way of life. Yet the influence of Native American culture is still felt throughout the United
States.Many of the foods commonly eaten in the United States today were first grown by
American Indians. Among these foods are corn, white and sweet potatoes, peanuts, pumpkins,
tomatoes, and certain beans and berries. Native Americans also discovered ways to use
herbs and other plants as medicines.
Native American inventions, such as canoes, ponchos and parkas（风雪衣），
hammocks（吊床）and snowshoes are still in use today. Other marks of Indian culture
were left in American place names. The state of Alaska, for example, takes its name from
a Native American word meaning“great land”.
Today nearly 1.5 million Native Americans live in the United States. Some still preserve
the ways of their ancestors, speaking Indian languages and keeping alive their legends,
art forms, and customs. Other Native Americans keep few of the old ways, but remain
proud of their people’s contributions to American history.
Americans have come to appreciate the beauty of Indian crafts（工艺）. Many artists
find inspiration in traditional Indian designs. Many Indian values, too, have become widely
held. Americans today believe that the land, air and water are precious resources (资源)
that must be carefully protected. As one government official wrote recently:We have
slowly come back to some of the truths that the Indians knew from the beginning. People
need to learn from nature, to keep an ear to the earth, and to refill their spirits in frequent
contacts with animals and wild land.
1. What does“Native Americans”refer to in the passage?______
A. Europeans settling in the Americas.
B. People born and living in America.
C. American Indians.
D. Ancestors of Americans.
2. What influences Americans most according to the passage?
A. Agriculture of Native Americans.
B. Ancient Indian medicines.
C. Native American inventions.
D. I ndian languages. .
3. The underlined word “inspiration” (in Paragraph 5) means ______.
A. confidence
B. creative ideas
C. power
D. good advice
4. Which of the following best shows the Indian values?
A. Americans should preserve the ways of Indian ancestors.
B. Native Americans are still making contributions to American history.
C. The resources offered by nature should be carefully protected.
D. Native Americans knew the truths long before Americans do today
：
，由此可知，粒子的运动的时间与粒子的速度的大小无关，所以粒子在磁场中的周期相同，由粒子的运动的轨迹可知，三种速度的粒子的偏转角分别为90°、60°、30°，所以偏转角为90°的粒子的运动的时间为 
，偏转角为60°的粒子的运动的时间为 
，偏转角为30°的粒子的运动的时间为 
,所以有 
,故选C。