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论文作者:佚名论文属性:短文 essay登出时间:2009-09-24编辑:lisa点击率:11095
论文字数:3000论文编号:org200909241536353376语种:英语 English地区:中国价格:免费论文
关键词:托福试题
Question 45-50
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