血睾屏障的组成中不含有（）。A.曲细精管间的结缔组织 B.曲细精管的基膜 C.支持细

真空中有两个点电荷，电荷量分别为+q₁和-q₂，两者固定于相距r的a、b两点上，有一个点电荷q放在a、b所在直线上的c点，且能静止不动，则下列说法正确的是（　　）

A．c点的电场强度为零

B．c点是a、b两点之间的某一位置

C．电荷q一定带正电

D．电荷q的电荷量必须为一确定值才能静止不动

某市一内资房地产开发企业2009年有关经营情况如下：
(1)1月1日与当地银行签订借款合同一份，合同记载借款金额1500万元，借款期限11个月，还款到期日11月30日；
(2)当年用借款1500万元和自有资金800万元，购得非耕地45000平方米的使用权用于开发写字楼和普通标准住宅楼；
(3)第一期工程主要是“三通一平”和写字楼开发，于11月30日竣工，按合同约定支付建筑承包商全部土地的“三通一平”费用500万元和写字楼建造成本7000万元。写字楼占地面积22000平方米，建筑面积70000平方米；
(4)到12月31日为止对外销售写字楼50000平方米，全部签了售房合同，每平方米售价0.32万元，共计收入16000万元，按售房合同规定全部款项于12月31日均可收回，有关土地权证和房产证次年为客户办理；其余20000平方米与某企业合作投资兴办五星级酒店，共担风险，该酒店2009年由于刚开业出现亏损，未分配利润。普通标准住宅楼尚未开发；
(5)在售房过程中发生销售费用1500万元；发生管理费用900万元。
(说明：计算土地增值税开发费用的扣除比例为10%)
根据以上资料和税法有关规定，回答下列问题：

征收土地增值税时应扣除的开发成本金额为( )万元。

A．6200.66

B．6100.2

C．7244.44

D．6000

电子邮件又称为（），是目前因特网上使用最广泛的应用服务之一。

A.ASCII

B.TCP/IP

C.电报

D.E-mail

施工预拱度在确定预挠度时，应考虑（）等因素。

A、卸架后上不构造本身及活载一半所产生的竖向挠度δ1

B、支架在荷载作用下的弹性压缩δ2

C、支架在荷载作用下的非弹性变形δ3

D、支架基底荷载作用下的非弹性沉陷δ4

Researchers investigating brain size and mental ability say their work offers evidence that education protects the mind from the brain’s physical deterioration.

(46) is known that the brain shrinks as the body ages, but the effects on mental ability are different from person to person. Interestingly, in a study of elderly men and women, those who had more education actually had more brain shrinkage.

"That may seem like bad news," said study author Dr. Edward Coffey, a professor of psychiatry and of neurology at Henry Ford Health System in Detroit.

(47) However, he explained, the finding suggests that education allows people to withstand more brain tissue loss before their mental functioning begins to break down..

The study, published in the July issue of Neurology, is the first to provide biological evidence to support a concept called the "reserve" hypothesis, according to the researchers. In recent years, investigators have developed the idea that people who are more educated have greater cognitive reserves to draw upon as the brain ages; in essence, they have more brain tissue to spare.

(48) Examining brain scans of 320 healthy men and women aged 66 to 90, researchers found that for each year of education the subjects had, there was greater shrinkage of the outer layer of the brain known as the cortex. Yet on tests of cognition and memory, all participants scored in the range indicating normal.

"Everyone has some degree of brain shrinkage," Coffey said. "People lose (on average) 2.5 percent per decade starting in adulthood."

There is, however, a "remarkable range" of shrinkage among people who show no signs of mental decline, Coffey noted. Overall health, he said, accounts for some differences in brain size. Alcohol or drug use, as well as medical conditions such as diabetes and high blood pressure, contribute to brain tissue loss throughout adulthood.

In the absence of such medical conditions, Coffey said, education level helps explain the range of brain shrinkage exhibited among the mentally-fit elderly. The more-educated can withstand greater loss.

(49) Coffey and colleagues gauged shrinkage of the cortex by measuring the cerebrospinal fluid surrounding the brain. The greater the amount of fluid, the greater the cortical shrinkage.

Controlling for the health factors that contribute to brain injury, the researchers found that education was related to the severity of brain shrinkage. For each year of education from first grade on, subjects had an average of 1.77 milliliters more cerebrospinal fluid around the brain. Just how education might affect brain cells is unknown.

(50) In their report, the researchers speculated that in people with more education, certain brain structures deeper than the cortex may stay intact to compensate for cortical shrinkage.

(47) However, he explained, the finding suggests that education allows people to withstand more brain tissue loss before their mental functioning begins to break down