One morning, a rabbit, a dog, a cat, and a duck went looking for food outside.  The rabbit says: "If I get food, the dog will also get food."  The dog says: "If I get food, the cat will also get food."  The cat says "If I get food, the duck will also get food."  In the evening, they find that all of them tell the truth but only two of them get food.~\uline{~~~~~~~~~~}~and~\uline{~~~~~~~~~~}~don\textquotesingle t get 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. 