[解析]
(1)C.细节理解题.阅读全文,根据文章内容,尤其是第一段Limiting kids' recreational(消遣) screen time to less than two hours a day,along with enough sleep and physical activity将孩子的娱乐屏幕时间限制在每天不到两小时,以及足够的睡眠和身体活动,可以提高认知能力.可知,本文是写给父母的,故选C.
(2)D.主旨大意题.阅读全文,根据文章内容可知,本文主要讲述了研究发现将孩子的娱乐屏幕时间限制在每天不到两小时,以及足够的睡眠和身体活动,可以提高认知能力.故选D.
(3)C.细节理解题.根据第三段Those who met all three had the most "superior" global cognition,followed by those meeting the sleep and screen time recommendation and finally the screen time recommendation alone,according to the study.可知,身体活动时间对认知的贡献最小.故选C.
(4)B.推理判断题.根据最后一段Besides,parents should also make sure to encourage physical activity and have bedroom rules such as creating "tech﹣free zones".可以推测出,美国组织建议创建"无科技区"是为了确保孩子有足够的睡眠时间.故选B.