B) it provided an exciting travel experience

　　C) its passengers could enjoy the great western outdoors

　　D) it was widely advertised in newspapers and magazines in Chicago and Seattle

　　Passage Four

　　Why does cram go bad faster than butter?Some researchers think they have the answer, and it comes down to the structure of the food, not its chemical compositiona finding that could help rid some processed foods of chemical preservatives.

　　Cream and butter contain pretty much the same substances, so why cream should sour much faster has been a mystery. Both are emulsionstiny globules (小球体)of one liquid evenly distributed throughout another. The difference lies in whats in the globules and whats in the surrounding liquid, says Brocklehurst, who led the investigation.

　　In cream, fatty globules drift about in a sea of water. In butter, globules of a watery solution are locked away in a sea of fat. The bacteria which make the food go bad prefer to live in the watery regions of the mixture. This means that in cream, the bacteria are free to grow throughout the mixture, he says.

　　When the situation is reversed, the bacteria are locked away in compartments (隔仓室)buried deep in the sea of fat. Trapped in this way, individual colonies cannot spread and rapidly run out of nutrients (养料). They also slowly poison themselves with their waste products. In butter, you get a self-limiting system which stops the bacteria growing, says Brocklehurst.

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