In November 2018, the U.S. Centers for Disease Control and Prevention warned that due to an outbreak of the E. coli bacteria, no romaine lettuce in the United States could be assumed safe to eat and that all of it should be discarded. Following this recommendation, sales of other leafy vegetables such as kale increased. Based on this information

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The increase in sales of kale following the recommendation to discard romaine lettuce suggests that consumers substituted kale for romaine lettuce, indicating that the two goods are substitutes and thus have a positive cross-price elasticity.