题目

A new research, presented on Monday, suggests that parents who go down slides with their kids are actually making slides even riskier for their little ones.Led by Dr. Charles Jennissen, a professor at the University of Iowa, the research found that placing children (especially infants and toddlers蹒跚学步的小孩) on adult laps increases the risk of injury to their lower legs, including broken bones.I've seen a lot of these injuries all through my career, and I hadn't seen anybody talk about this problem, says Jennissen.So Jennissen and his colleagues found that about 350,000 children und如图，多面体OABCD，AB=CD=2，AD=BC= ，AC=BD= ，且OA，OB，OC两两垂直，则下列说法正确的是（ ） A.直线OB∥平面ACDB.球面经过点A，B，C，D四点的球的直径是 C.直线AD与OB所成角是45°D.二面角A﹣OC﹣D等于30°