What do these two changes have in common?
a piece of avocado turning brown
milk going sour

 - Both are caused by cooling.
 - Both are caused by heating.
 - Both are chemical changes.
 - Both are only physical changes.
Step 1: Think about each change.
A piece of avocado turning brown is a chemical change. The avocado reacts with oxygen in the air to form a different type of matter.
If you scrape off the brown part of the avocado, the inside will still be green. The inside hasn't touched the air. So the chemical change hasn't happened to that part of the avocado.
Milk going sour is a chemical change. The type of matter in the milk slowly changes. The new matter that is formed gives the milk its sour taste.
Step 2: Look at each answer choice.
Both are only physical changes.
Both changes are chemical changes. They are not physical changes.
Both are chemical changes.
Both changes are chemical changes. The type of matter before and after each change is different.
Both are caused by heating.
Neither change is caused by heating.
Both are caused by cooling.
Neither change is caused by cooling.