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.
Why will the next 50 years be critical for the world population？

A. Because by 2050, the population growth will have leveled off.
Because the world will be unable to provide enough food for such a big population.
C. Because the population will reach a considerable large size.
D. Because by that time it'll be too late to cut population growth rate.