Some people believe that they would be better off more adaptive activities in their leisure time, such as establishing relationships and teaching their children．A.doingB.to doC.having doneD.to be doing
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A解析这里考查的是现在分词做条件状语的用法，句意是：一些人相信他们会更幸福的，如果在空闲时间多做些适应性活动，如建立关系，和教育孩子。选A。