问题 填空题

下面是一些正在振动着的物体:

甲:手臂以2s 2次上下挥动、

乙:蝙蝠的嘴发出频率为100000Hz的振动、

丙:被小锤敲击的音叉发出的频率为256Hz的声音、

对以上物体的一些说法,正确的是______.

A、甲、乙、丙都是声源,人耳都能听到它们发出的声音

B、丙是声源,人耳能听到它发出的声音,甲、乙不是声源,人耳听不到声音

C、甲、乙、丙都是声源,人耳只能听到丙发出的声音.

答案

(1)甲、乙、丙都是物体在振动,它们都能发出声音,因此都属于声源;

(2)频率是指物体每秒振动的次数,甲为1Hz,乙为100000Hz,丙为256Hz,人的听觉范围是20-20000Hz,只有丙的频率在人的听觉范围之内,因此对照四个选项可以看出,只有C是正确的.

故答案为:C.

多项选择题
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Money, jewels and important documents are not the only valuable placed in banks these days. Some mothers store their breast milk in banks.

There’re ten breast banks set up across the United States, where mothers can donate their extra milk for other women’s babies. Experts say breast milk is the best food for babies. The World Health Organization says it is the only food babies should get during the first six months of life, in most cases. Breast milk is especially important for babies born too early. Sometimes these premature babies must stay in the hospital for many weeks.

James Cameron is a doctor who treats newborns at Lutheran Children’s Hospital in Fort Wayne, Indiana. He says breast milk is almost like medicine. Doctor James Cameron said, “The fact is that there are so many different proteins and specialized sugars in the breast milk that the mom’s able to make that help provide immunity. It’s very important for the health of the newborn.”

There are several reasons some mothers may not be able to breastfeed. Some are not able to produce enough milk. Others might be taking medicines or have medical problems that prevent the process.

Lucy Baur lives near Fort Wayne. She fed her milk to both her children and always had more milk than they needed. She wanted to donate to the Indiana Mothers Milk Bank in Indianapolis. But freezing and shipping milk can be costly. Then, a donor station opened near her home.

Milk donations in the United States work like this: Donors must be willing to provide almost three liters of breast milk. They freeze the milk and take it to the station. There, employees warm the milk and mix it with other mothers’ milk. Then, the milk is heated to kill bacteria. After that, the technicians test samples of all the milk to make sure it is safe and healthful. The milk is re-frozen and sent to the main milk bank. The milk bank transports the milk to hospitals to feed premature or sick babies. Donors are tested for diseases before any milk is accepted. They are not permitted to smoke tobacco, use illegal drugs or drink too much alcohol.

小题1:The purpose of setting up breast milk banks is to_____.

A.store extra fresh milk for school children

B.provide milk to children who go hungry

C.help mothers donate milk to other babies

D.offer free breast milk to premature babies小题2:The breast milk is important for newborns because_____.

A.it protects them from any possible disease

B.it contains rich nutrients that they need

C.it is the only food that babies can eat

D.it benefits them as the best medicine小题3:The underlined word “they” in the last paragraph refers to____.

A.milk bank employees

B.milk bank doctors

C.milk bank nurses

D.breast milk donors小题4:Which of the following shows the process of milk donations in the USA?

a. The mixed breast milk is heated to kill bacteria.

b. Breast milk is donated by mothers, frozen by them and sent to the stations.

c. The breast milk samples are tested for safety reason.

d. Different donors’ breast milk is warmed and mixed together by the station.

e. The milk is frozen again and sent to the main milk bank.

A.bdace

B.bcdea

C.badec

D.cbaed