Task: Question Answering
Input (English): One type of cold remedy is an effervescent tablet that breaks down in water. When the tablet is placed in water, it forms gas bubbles. In this example, the formation of a gas  
 (A) is a physical property. (B) occurs when adding water to acid. (C) is a sign of a chemical change. (D) occurs when the water boils.
Output (English): C