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问题:

What have mobile phones become according to resear

答案:

The backbone of modem social life. The backbone of modem social life. 解析:(由第二段第二句“对手机使用者的研究表明手机已不再仅作为实用的沟通工具,而是成为the backbone of modern social life”,可知答案为The backbone of modem social life。)