从第三段第五行开始说发达国家的情况“A declining birth rate may lead to unemployment…，Cities with a declining population may have to face the prospect of a shrinking tax base and a fall in land values. . . teachers may be thrown out of work. . . prices also decline and the building industry is weakened…”，所介绍的这一切都是经济问题，所以说控制人口数量对发达国家而言，对其经济会有不利影响，故选C。