问题 解答题

龙凤胎是由两个卵细胞和一个精子结合发育而成的.______.

答案

在亲代的生殖细胞形成过程中,经过减数分裂,两条性染色体彼此分离,男性产生两种类型的精子--含X染色体的精子和含Y染色体的精子.女性则只产一种含X染色体的卵细胞.受精时,如果是含X的精子与卵子结合,就产生具有XX的受精卵并发育成女性;如果是含Y的精子与卵子结合,就产生具有XY的受精卵并发育成为男性.这说明男女的性别在受精卵形成时就已确定.龙凤胎,是一男和一女性别不同,因此两个孩子来自于两个受精卵.

故答案为:×

阅读理解

阅读理解。

     A 69yearold grandmother with  no teeth of her own has eventually won a long legal  battle to stop a

Scottish regional council (政务委员会) adding fluoride (氟化物) chemical to the public water  supply.

     In a case which has already cost the taxpayer £1,000,000,the judge ruled that it was beyond the

powers of the local authority to add the chemical to the water in order to reduce tooth decay.

     At her home last night Mrs Catherine McColl said, "I did what I thought was right and I would do it

again, too." She claimed that adding fluoride to public drinking water made it into some kind of dirty

soup ."Where would it stop?"she asked. "They might come up with the idea of putting drugs into the

water to keep the unemployed quiet."It was a horrible poison, she said, that could have caused all kinds

of diseases, including cancer.

     The judge, however, concluded that there was no evidence  to suggest that the inclusion of fluoride in

the water supply would have had a negative effect on public health. Although the chemical might serve as

an efficient and convenient means of achieving a beneficial effect on  the dental health of consumers

generally, he said, and its  use was greatly favoured by the dental profession, he  could also understand

why some members of the public, Mrs McColl in particular, might be passionately opposed  to the

action of the Water Authority in assuming the  right to improve public wellbeing without consulting the

public in the first case. The Authority's legal duty to provide "wholesome" water for public consumption

which was both safe and pleasant to drink ,did not,he said, extend to their right to safeguard public health

by chemical means.

1. Mrs McColl felt so strongly about the fluoride issue that she eventually ________.

A. took the local council to court

B. had a physical fight with the judge

C. urged the authority to apologize

D. spent much money removing the chemical

2. According to what the judge said in the passage, adding fluoride to the water________.

A. was not proved to be harmful

B. was the duty of the local authority

C. was strongly opposed by dentists

D. was surely beneficial to the public

3.The word "wholesome"in the last paragraph can be best replaced by the word"________".

A. clear    

B. poisonless

C. healthy   

D. recycled

4. From the passage we learn that people like Mrs McColl are more concerned about________.

A. the improvement of their personal health

B. the problem of unemployment in their community

C. the chemicals to be used for the improvement of water quality

D. their right to be informed of the authorities' decisions

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