下列事件中，是必然发生的事件是（  ）

一盒装牛奶，标有250mL的体积. 若该牛奶密度是 1.2×10³kg/m³，合____g／cm³，则牛奶的质量为____g. 喝掉一半后，牛奶的密度将____（选填“变大”、“变小”或“不变”）.

下列语句中，划线词语使用不正确的一项是(　 　)

   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.