给下列句子选择合适的答语。
① It's 7 o'clock ____ A. It's time for English class.
② It's 9 o'clock ____ B. It's time for breakfast
③ It's 5 o'clock. ____ C. It's time to go home.
千姿百态的物质世界中，各种物质之间存在着多种相互作用，也不断地发生着变化。下列生活中常见的变化中，一定发生化学变化的是
