Two identical coolers are packed for a picnic. Each cooler is packed with eighteen 12 -oz soft drinks and 3 lb of ice. However, the drinks that went into cooler A were refrigerated for several hours before they were packed in the cooler, while the drinks that went into cooler B were at room temperature. When the two coolers are opened three hours later, most of the ice in cooler $A$ is still ice, while nearly all of the ice in cooler B has melted. Explain.