相对于其他生殖方式而言，胎生的优势在于（ ）
A．降低出生个体的数量
B．提高了后代的成活率
C．增强了后代的体质
D．使后代的数量大大增加
哺乳动物在繁殖期间哺乳动物雌雄交配，雄性的精子进入雌性的体内，和卵细胞结合，形成受精卵，在雌性动物的子宫内发育形成胚胎，胚胎在母体的子宫内，通过胎盘和母体之间进行物质交换，发育成胎儿，胎儿从母体生出来，这种生殖方式叫胎生．胎生减少了后代对环境的依赖，刚出生的幼体身上无毛，眼睛没有睁开，不能行走，只能靠母体的乳汁生活，叫哺乳，所以称为哺乳动物，胎生哺乳大大提高了后代的成活率，增强了对陆上生活的适应能力．
故选B．
| The first 50 years of the next millennium will be critical for the world’s population. By 2050 population growth should have leveled off, but by then we’ll have 10 billion people -- two-thirds as many again as we have today. The rate of population growth is something we can choose right now, though it’s not something that just happens, but a matter of human choice. The choice is a complicated one, with many variables, but it remains a choice. If we want to prevent a population explosion, we should take action now -- or assist the poorer countries to do so. They need better government, better institutions, better labor and capital markets, better schools. Anything that increases the value of women’s time and adds to the cost of caring for a child makes a woman less likely to have that child. Since big families are often seen as safety nets for illness and old age, improving poor people’s access to insurance, pensions and welfare institutions also has a major impact. This can be as simple as rural credit, providing a means of saving. Finally, there is education -- both for women and, perhaps even more important, for the next generation of children. These steps are there to be taken, but there appear to be some countries that are not seriously trying at the moment. If we cannot achieve that we will certainly not control population. That said, I don’t feel pessimistic that we are going to run out of resources: we are becoming more efficient at producing food faster than the rate at which population is increasing. There is, however, a risk that we will wreck the environment so effectively that the world will no longer be an attractive place to live. That really would be a dismal outcome, to reach world population equilibrium only to find we’d destroyed the natural environment in the process. |
By 2050, the population growth will ______.
A．rise steeply
B．fall sharply
C．come to a halt
D．remain even