Questions 24 to 27 are based on the following passage. 

Humans have long turned to the dog for its nose,especially in its ability to hunt, track missing people, andsearchfor drugs- But there is a new challenge： Bomb-detecting dogs now have to learn to find the increasinglycommon Improvised Explosive Devices (IEDs)( 临时组装的爆炸装置） that can be assembledfrom materials notdangerousby themselves.“ So we’ re now asking dogs not just to find a needlein ahaystack (干草堆）一 now theproblem is more like saying to the dog we needyou to find any sharpobject in the haystack, ” says Clive Wynne，aprofessor atArizona State University. 

Wynne directs the study that develops methods to train dogs to identify a wide variety of ingredients thatcould be usedto make bombs. The dogs not only have to detect whether explosive materials are present,they alsohave to determine if the chemicals they smell could combine to form. an explosive mixture. The dogs needto learnthat when combined with other elements, even sugar could be usedin abomb. “ we want the dogs to grasp theconcept of ‘ this might explode ’”. 

Wynne says the big problem is that IEDs arejust that---improvised and completely unsteady.A dog ’s nosecanbe over a thousand times more powerful than a human ’s nose. Some agenciesspent billions of dollars over thelast 15 yearstrying to createatechnical replacement for the noseof the dog, and at the end of all that they just hadto admit “ No, we cannot get close ”. Wynne thinks his team candevelop bomb sniffing (嗅，闻）dogsthat can detectIEDs at a small part of that price. All it takes is patience. 

24.According to the passage， the new task of bomb-detecting dogs is to learn______.