[求助] one interesting question

In a monogamous culture, 90% of the adults are married. The average number of children per family is five and over-population is a threat. Programs to encourage birth-control have been ineffective. It has been suggested that this failure is due to these programs ignoring a tradition that values male children very highly, so that every parent wants to have at least one son. It is proposed that couples be encouraged to use birth-control measures after the birth of their first son. If this proposal is widely accepted in the culture, we may expect that:

A) the rate of population increase will be slowed, and future generations will contain a disproportionately high number of females.

B) the rate of population increase will be slowed, and the gender balance in future generations will remain as it is at present.

C) the rate of population growth will remain the same, and future generations will contain a disproportionately high number of females.

D) there will be no significant effect either on population growth or on gender balance.

E) the population will decline precipitously, because approximately half of all families will have only a single child.

the answer is B,mine is E

ok, i understood why E is incorrect, but why B is correct?

the gender balance in future generations will remain as it is at present.

how to deduce that the gender balance will remain as it is at present? surely it will have some change aften the born-control policy?

thank you

b/c the possibility of have a son or a girl is always 1/2.  Before the government's recommendation, if he wanted three sons, then probably he would have ended up with six kids, 3 girls and 3 boys.  Now, as a good citizen, he would follow the government's guideline, which will lead to 2 kids, i.e. 1 girl and 1 son.  Ultimately, the ratio of boys to girls will remain unchanged, that is, 1:1.

follow the policy, the end is that every family will have a son, say, the first is a girl, the second is a boy, ok, they stop.

but not all family will have a girl, say, the first is a boy, then they stop.

since the possibility to have girls and boys is 50% respectively, i think the gender banlance will be broken

TO SUM UP. 一家至少有一个男孩，却不一定再有女孩，人口比例肯定会变。

晕

To give an example, a family which originally wanted to have three sons would have had two situations: a) girl,boy,girl,boy.girl,boy,stop

                b)boy,gril,boy, girl,boy.stop

Later on, by following the guideline, the situations would be the same:

a)girl,boy,stop

b)boy, stop

Looking closely,  on average, no matter before or after, a family should have 0.5 more boy than girl.  That is, the ratio of boy:girl would not be changed.

Hope that helps