4．
OtolithsFish have ears．Really．They're quite small and have no opening to the outside world carrying sound through the body．For the past seven years，Simon Thorold，a university professor，has been examining fish ears，small round ear bones called otoliths．
As fish grow，so do their otoliths．Each day，their otoliths gain a ring of calcium carbonate （ 碳酸钙）．By looking through a microscope （ 显微镜） and counting these rings，Thorold can determine the exact age of a young fish．As a fish gets older，its otoliths no longer get daily rings．Instead，they get yearly rings，which can also be counted，giving information about the fish's age，just like the growth rings of a tree．Ring counting is nothing new to fish scientists．But Thorold has turned to a new direction．They're examining the chemical elements （ 元素） of each otolith ring．
The daily ring gives us the time，but chemistry tells us about the environment in which the fish swam on any given day．These elements tell us about the chemistry of the water that the fish was in．It also says something about water temperature，which determines how much of these elements will gather within each otolith ring．
Thorold can tell，for example，if a fish spent time in the open ocean before entering the less salty water of coastal areas．He can basically tell where fish are spending their time at any given stage of history．
In the case of the Atlantic croaker，a popular saltwater food fish，Thorold and his assistant have successfully followed the travelling of young fish from mid-ocean to the coast，a journey of many hundreds of miles．
This is important to managers in the fish industry．They know nearly nothing about the whereabouts （ 行踪） of the young fish for most food fish in the ocean．Eager to learn about Thorold's technology，fish scientists are now lending Thorold their ears．
52．What can we learn about fish ears from the passage？D
A．They are quite small soft rings．
B．They are not used to receive sound．
C．They are openings only on food fish．
D．They are not seen from the outside．
53．Why does the writer compare the fish to trees？B
A．Trees gain a growth ring each day．
B．They both have growth rings．
C．Their growth rings are very small．
D．Trees also have otoliths．
54．Why is it important to study the chemistry of otolith rings？C
A．The elements of the otoliths can tell the history of the sea．
B．Chemical elements of otoliths can tell how fast fish can swim．
C．We can know more about fish and their living environment．
D．Scientists can know exactly how old a fish is from the elements．
55．What does the underlined sentence"ffish scientists are now lending Thorold their ears．"mean？A
A．Fish scientists are very interested in Thorold's research findings．
B．Fish scientists want to know where they can catch more young fish．
C．Fish scientists lend their fish to Simon Thorold for chemical studies．
D．Fish scientists wonder if Thorold can find growth rings from their ears．
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