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问题 选择题

下列哪些因素能诱发遗传物质发生改变？（　　）

A．宇宙射线的辐射作用

B．不同品种生物之间的杂交

C．给植物大量施肥

D．用一定浓度的化学药剂处理植物幼苗

答案

把萌发的种子拿到太空去经过射线辐射，再回地面种植而后长出新品种，是使植物细胞体内基因发生突变的结果，属于基因突变．因此，宇宙射线的辐射作用，能诱发遗传物质发生改变；不同品种生物之间的杂交，给植物大量施肥，用一定浓度的化学药剂处理植物幼苗，遗传物质均没有改变；

故选：A

单项选择题

清除受热面上的积灰，保持受热面清洁以保证传热正常进行的设备是（）。

A.吹灰器；

B.碎渣机；

C.仓泵；

D.罗茨风机。
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填空题



Most taxpayers have had enough income tax collected by their employers during the year. So they do not (36) any money. In fact, most Americans get some money back. Last year, the Internal Revenue Service returned at least some money to more than (37) Americans paying individual income tax.
The United States has what is called (38) . Tax rates increase as (39) increase. This year, people earning more than (40) are taxed at the highest rate.
Earnings above that amount are taxed at (41) . Earnings below that amount are taxed at lower rates.
Individuals who earn less than (42) pay no income tax. But they do pay (43) , Medicare and other taxes.
There are many different ways for people and businesses to reduce their (44) Most homeowners, for example, can reduce their taxes a little by reporting to the I.R.S. the interest they pay (45) . This is called a (46)
In 2004, personal income taxes provided the government with most of its money: (47) of the budget. Social security and other (48) taxes provided (49) . Other forms of income include (50) , money borrowed to (51) and special taxes on (52) .
But income from taxes did not provide enough to pay for government spending. The I.R.S. says the budget deficit for 2004 was about (53) dollars.
Preparing tax documents can be complex. The I.R.S. estimates that taxpayers need an average of about 13 hours to prepare tax documents. And that is just for the basic tax form. For business, the I.R.S. estimates an (54) needs more than 50 hours. This is why a lot of Americans pay (55) to complete their tax documents for them.
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