On Thursday Morning, Charles Prepared His Typical Breakfast of Cornflakes

On Thursday morning, Charles prepared his typical breakfast of cornflakes with milk and a cup of coffee. However, instead of having guava with his breakfast, he tried eating grapefruit for the first time. At lunchtime he ate his typical lunch of tuna salad and potato chips. However, instead of having a Coke to drink with his lunch, he drank a new fruit-flavoured soft drink for the first time. During the mid-afternoon he became extremely ill. If his illness causes him to develop a conditioned response to something, which stimulus is likely to be the conditioned stimulus?

A) grapefruit, because it was a novel stimulus and was the first novel thing he ingested that day
B) tuna, because it is most likely have been sour and it was ingested closest in time to the onset of the illness
C) milk, because the milk may have been sour and it was ingested first thing in the morning
D) the new fruit-flavoured soft drink, because it was a novel stimulus and was ingested closest in time to the onset of the illness