阅读下列短文，从每题所给的A、B、C、D四个选项中，选出最佳选项。
C
The government in China plans to end its one-child policy (政策) and let families have two children instead. The plan was meant to balance (平衡) population development and stop a falling birth rate (出生率).
China started the one-child policy in the 1980s. But the government allowed only a small number of couples to have two children. For example, some families in the countryside could have two children, if the first-born child is a girl. In 2013, the Chinese government gave other couples a chance to have two children if one of them was an only child.
Jiang Quanbao, a population expert, explained how Chinese families react to the newest policy. "Lots of young people in the cities are no longer interested in having a second child," he said.
At the end of 2014, China had a population of 1.37 billion people. But the labor market population will drop in 35 years. That is to say, the country will be in great need of labor by the year 2050. With the "two children" policy, an increase in births can solve this problem.
时，它是矩形 ④当AC=BD时，它是正方形
