On a fine morning, a rabbit, a dog, a cat, and a duck went out to look for food.  The rabbit said: "If I get food, the dog will also get food."  The dog said: "If I get food, the cat will also get food."  The cat said: "If I get food, the duck will also get food."  That evening, they found that all of them were telling the truth but only two of them did get food.~\uline{~~~~~~~~~~}~and~\uline{~~~~~~~~~~}~didn\textquotesingle t get any food. 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
We can infer that if the rabbit gets food, then all of the other three would get food; if the dog gets food, then both of the cat and duck would get food. Therefore, only when the rabbit, and the dog don\textquotesingle t get food, the cat and the duck would get food. 