The most surprising thing was that the skin bacteria of dog owners from different households were almost similar as if they'd cohabited without a dog. Put another way, you and a friend across town who both own dogs share a skin ecosystem as alike as a married couple does.

　　The reason for this appears to be that dogs harbour some kinds of bacteria that are rare on human skin. One in particular, a family of bacteria called Methylophilaceae, was abundant in the mouths of dogs sampled as well as the skin of their owners—indicating a common occurrence of oral－skin transfer between dogs and their owners, according to the results published in the journal eLife.

　　21．From the study, we know that ________．a live on our skin because of dog ownership

　　B．nobody finds it surprising that there should be some germ－exchanging between dogs and their ownersthere would most probably be no microbe on our skin if it were not for dogs

　　22．The research took the following factors into consideration EXCEPT ________．lds

　　B．families with children or households pets

　　C．families without children or households pets

　　D．dogs being the only pets in the households

　　23．Why did researchers analyse the DNA of the bacteria they found?

　　A．To search for the way microbes pass from one member of a family to another.

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