If you have ever wondered how an elephant smells, scientists have the answer. Researchers have discovered that African Elephants

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问题     If you have ever wondered how an elephant smells, scientists have the answer. Researchers have discovered that African Elephants have the largest number of genes【C1】________smell of any mammal. Given the size of their trunks, and how important it is【C2】________their survival, it is probably unsurprising that an elephant’s nose is not only the longest in the animal kingdom, but also the most effective.
    In【C3】________, humans and other primates have a poor sense of smell. "The functions of these genes are not well known,【C4】________they are likely important for the living environment of African elephants," said author Dr Yoshihito Niimura of the University of Tokyo. "【C5】________, an elephant’s nose is not only long but also superior. " The sense of smell is【C6】________to all mammals, and they use it for【C7】________food, finding mates and locating their offspring.
    In a study published in Genome Research, scientists【C8】________the 13 mammal species and found that African Elephants have twice the number of smell genes【C9】________dogs and five times more than humans. They have around 2,000 genes alone that are associated with scent. Humans in comparison have just under 400 and some other primates even【C10】________. The study found that 20,000 genes are【C11】________for the sense of smeD in mammals, of which around half are functional, but the collections【C12】________for each species. Horses have around 1,000 smell genes, rabbits around 750 and rats about 1,200.
    "The large repertoire of elephant (smell) genes might be attributed to elephants’ heavy【C13】________on scent in various contexts, including social【C14】________and reproduction," added Dr Niimura. African and Asian elephants possess a【C15】________scent organ behind each eye, and male elephants can produce a special substance during annual mating, which is characterized【C16】________increased aggressiveness. And【C17】________studies have revealed that, African elephants can reportedly【C18】________between two Kenyan ethnic groups—the Maasai, whose young men【C19】________manhood by spearing elephants, and the Kamba, who are agricultural people that【C20】________little threat to elephants through smell.
【C4】

选项 A、so
B、because
C、and
D、but

答案D

解析 前一个分句The functions…意为“这些基因并不为人所熟知”,后一个分句意为“它们对于非洲象的生存环境很重要”。可以看出,两者为转折关系,故选D项but“但是”。A项so“那么”表结果。B项because“因为”表原因。C项and“和”表并列。
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