Today is Saturday. It’s sunny and warm. Many children from No. 1 Middle School are at the Old People’s Home. Now, they’re helping the old people.
   Look! Li Min and Zhan Ran are cleaning the windows. Ma Lan and Liu Qian are washing clothes.
   What’s Li Na doing over there? Many old people are clapping their hands(鼓掌)for her. Oh,  she’s singing for them. Her song is really beautiful. And where’s the American boy, Tom? He likes helping others and he often comes here. Listen! Who’s speaking English? It’s Tom. He’s teaching two old people to speak English.
   All the children and the old people are very happy. “We are happy when we help others, ” says Tom.

如图，在△ABC中，D、E分别是AB、AC的中点，若BC=10，则DE= ____．

实验室中有两瓶失去标签的稀盐酸和氯化钠溶液，下列四种试剂中能将其鉴别出来的是（  ）

你能告诉我4.20万精确到什么位吗？（   ）