1996年10月托福阅读全真试题

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

　　When a bloodhound trails a human being， what does it actually smell？ The human body， which consists of about 60 trillion living cells， sheds exposed skin at a rate of 50 million cells a day. So even a trail that has been dispersed by breezes may still seem 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 the bacteria working on both substances is another matter. One microbiologist estimates the resident bacteria population of a clean square centimeter of skin on the human shoulder at "multiples of a million." As they go about their daily business breaking down lipids， or fatty substances， on the skin， these bacteria release volatile substances that usually strike the bloodhound's nose as an entire constellation of distinctive scents.

　　45.What does the passage mainly discuss？

　　（A） Why people choose bloodhounds for household pets

　　（B） How a bloodhound's sense of smell works

　　（C） How humans compensate for an underdeveloped sense of smell

　　（D） The way in which bacteria work on skin cells and body sweat.

　　46.The author compares a scent molecule with a

　　（A） key

　　（B） lock

　　（C） cell

　　（D） bloodhound

　　47.In line 9， the word "it" refers to

　　（A） bloodhound

　　（B） human being

　　（C） smell

　　（D） body

　　48.According to the passage， how many cells of skin does the human body rid itself of every day？

　　（A） 60 trillion

　　（B） 50 million

　　（C） 1 million

　　（D） Between 31 and 50

　　49.In line 13， the word "rich" is used to mean that a trail is

　　（A） paved with precious materials

　　（B） a profitable business to get into

　　（C） a very costly undertaking

　　（D） filled with an abundance of clues.

　　50.Which of the following acts as a stimulus in the production of the human scent？

　　（A） Sweat

　　（B） Dead skin cells

　　（C） Bacteria

　　（D） Fatty substances.