14

【解析】动词辨析。原文表达：Sixty toddlers were each  14   to an adult tester holding a plastic container. 只有C选项be introduced to sth.表示“初次认识某事物”，符合文意。A 选项added “添加”；B选项transferred“转移”；D选项entrusted “委托”，皆不符合文意。