根据短文内容，回答下列问题。
   Linda and Mary are good friends. Linda is thirteen. She comes from the U.S.A. Mary is twelve. She's from London(伦敦). They study in the same school, but in different classes. Linda's father teaches English in a middle school. Her mother drives a car. Mary's mother works in an office. Her father is a doctor. Linda can speak English, French(法语) and a little Chinese. Linda's mother likes cooking. She likes eating rice. But Linda' s father doesn't. He likes drinking milk and eating bread. Mary's mother doesn't like cooking. But she likes doing some shopping. She can make nice skirts and dresses. Mary's father likes doing housework(家务).

1902年美国已有发电厂3621座，总装机容量121．2万千瓦；1907年德国已有三分之一的企业使用电动机。这预示人类进入到（    ）