3.  "We go through life surrounded by protective convoys of others," says Robert Kahn, a psychologist of the University of Michigan who studied the health effects of companionship.
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【正确答案】 1、studied→has studied。时态平行。全句的主语是现在时，定语从句介绍Robert做过的研究项目，所以用现在完成时比较合适。    
【答案解析】