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

　　(Line5) 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

　　(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 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

　　(Line15) 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 7, 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 10, 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

　　參考答案：BAABDC