What do these two changes have in common?
burning a marshmallow over a campfire
rust forming on a bicycle frame

 - Both are only physical changes.
 - Both are caused by heating.
 - Both are chemical changes.
 - Both are caused by cooling.
Step 1: Think about each change.
Burning a marshmallow is a chemical change. The heat from the fire causes the type of matter in the marshmallow to change. The marshmallow becomes black and crispy.
Rust forming on a bicycle frame is a chemical change. Oxygen in the air reacts with iron in the bicycle frame. The outside of the frame turns into a different type of matter called rust. Rust is reddish-brown and falls apart easily.
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.
Burning is caused by heating. But rust forming on a bicycle frame is not.
Both are caused by cooling.
Neither change is caused by cooling.