It is Sunday morning，Helen and her brother David are 1a walk in the park. There are lots of signs in the park. Helen knows a lot 2 them. But David 3. Suddenly Helen sees a boy climbing a tree. She says to the boy,"Don't climb the tree. Can't you see the 4Over there?" The boy jumps off the tree soon. David points to the sign and says to Helen，"So the sign means we 5climb trees，right?"  "Yes." says Helen.

据某网上商城数据显示，随着雾霾天气长期大范围持续，空气净化器日益走进普通百姓家庭，售价上涨的走势成为该行业发展的催化剂。不考虑其他因素，能正确反映这种变动传导效应的是（注：D、S为变动前，D1、S1为变动后）（  ）

如图所示，观察，伸肘时，肱二头肌和肱三头肌所处的状态是（  ）

古希腊的圣贤先哲从客观实际出发，俯察人生，对人的价值提出了许多闪耀着人文主义光芒的阐述，其中苏格拉底的主要观点是（    ）