Shiyun trained a group of rats that a tone predicts food pellets. Then she trained them that a light predicts the tone. She's about to test the rats to see if they learned the higher-order association and comes to you for advice on which type of conditional response to measure. Which of the following do you tell her would be the best measure? 

• Approach to tone
• Approach to goal location (food-delivery port) in response to tone
• Approach to goal location (food-delivery port) in response to light
• Approach to light Escape from the tone