The world's population continues to grow. There now are about 4 billion of us on the earth

. That could reach 6 billion by the end of the century and 11 billion in another 75 years. Experts long have been concerned about such growth. Where will we find the food, water, jobs, houses, schools and health care for all these people？

A major new study shows that the situation may be changing. A large and rapid drop in the world's birth rate has taken place during the past 10 years. Families generally are smaller now than they were a few years ago. It is happening in both developing and industrial nations.

Researchers said they found a number of reasons for this. More men and women are waiting longer to get married and are using birth control devices and methods to prevent or delay pregnancy. More women are going to school or working at jobs away from their homes instead of having children. And more governments, especially in developing nations, now support family planning programs to reduce population growth. China is one of the nations that has made great progress in reducing its population growth.

China has already cut its rate of population growth by about one half since 1970. China now urges each family to have no more than one child. And it hopes to reach zero population growth, the number of births equaling the number of deaths, by the year 2000.

Several nations in Europe already have fewer births than deaths. Experts said that these nations could face a serious shortage of workers in the future. And the persons who are working could face much higher taxes to help support the growing number of retired people.

In the sentence "Experts long have been concerned about such growth" of Paragraph 1, the phrase "concerned about" is similar in meaning to ______.

A．worried about

B．related to

C．engaged in

D．made a study of