¡¡¡¡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.
