托�？荚噦淇碱}和答案

時(shí)間：2020-12-28 12:31:01 托�？荚� 我要投稿

托�？荚噦淇碱}和答案

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托�？荚噦淇碱}和答案

　　Bloodhounds are biologically adapted to trailing their prey. The process by whichthe nose recognizes an odor is not fully understood, but there are apparently specificreceptor sites for specific odors. In one explanation, recognition occurs when a scentmolecule fits into its corresponding receptor site, like a key into a lock, causing a(Line5) mechanical or chemical change in the cell. Bloodhounds apparently have denserconcentrations of receptor sites tuned to human scents.

　　When a bloodhound trails a human being, what does it actually smell? The humanbody, which consists of about 60 trillion living cells, sheds exposed skin at a rate of 50million cells a day. So even a trail that has been dispersed by breezes may still seem(Line10) rich to a bloodhound. The body also produces about 31 to 50 ounces of sweat a day.

　　Neither this fluid nor the shed skin cells have much odor by themselves, but thebacteria working on both substances is another matter. One microbiologist estimatesthe resident bacteria population of a clean square centimeter of skin on the humanshoulder at "multiples of a million." As they go about their daily business breaking(Line15) down lipids, or fatty substances, on the skin, these bacteria release volatilesubstances that usually strike the bloodhounds nose as an entire constellation ofdistinctive scents.

　　45. What does the passage mainly discuss?

　　(A) Why people choose bloodhounds for

　　household pets

　　(B) How a bloodhounds sense of smell works(C) How humans compensate for an

　　underdeveloped sense of smell

　　(D) The way in which bacteria work on skin cellsand body sweat

　　46. The author compares a scent molecule with a(A) key

　　(B) lock

　　(D) bloodhound

　　47. In line 7, the word "it" refers to

　　(A) bloodhound

　　(B) human being