Here's a problem to solve: Which is an example of a physical change?

Among the 4 following options, which is the correct answer?

- A: breaking an object into smaller pieces by using a hammer
 
- B: allowing an iron chain to remain in salt water until it rusts
 
- C: heating a brown substance until it changes into a gray powder
 
- D: reacting vinegar and baking soda to make white foam and a gas
 