For thousands of years，people have lived with dogs．Ancient paintings on the walls of caves show people living with dogs．Almost a third of the homes in the United States and England have dogs，and these dogs come in a wide variety of shapes and sizes．
The largest dog in the world was a dog named Zorba．When Zorba，a mastiff，was seven years old in 1989，he was 94centimeters （37inches） tall．In other words，Zorba was more than half as tall as an adult man．At his largest，Zorba weighed more than a heavy weight boxer at 156kilograms（343pounds）．
In comparison，the smallest dog was a Yorkshire terrier from England．This dog was only the size of a matchbox，measuring 7.1centimeters（2.8inches） tall and 9.5centimeters （3.75inches） from nose to tail．It weighed about half as much as a common book，and even a young child could easily pick it up with one hand．It passed away in 1945when it was only two years old．
Some dogs are wonderful，not for their sizes，but for their brains．One incredible （难以制信的）dog is Endal，the companion （陪伴） of a man named Allen Parton who has used a wheelchair since a car accident in 1991．
In 2001，Parton was hit by a car while crossing a road with Endal，and thrown out of his chair．Endal quickly moved Parton into the recovery place，covered him with a blanket（毯子），and pushed his mobile phone close enough for him to reach．Then，once he saw that Parton was all right，Endal ran back and forth to a nearby hotel，barking until people came out to help．
Endal was rewarded for his bravery by being awarded a medal，and he has been the subject of a number of TV documentaries．

51．Why do people have dogs as pets？
A．Dogs used to live in caves．
B．Dogs are easy to be taken care of．
C．Dogs come in all shapes and sizes．
D．The passage doesn't tell us about it．
52．The underlined word"mastiff"in Paragraph 2probably means ．
A．a kind of painting
B．a kind of dog
C．a place in England
D．a kind of accident
53．The world's smallest dog weighed half as much as according to the passage．
A．a child's hand
B．a matchbox
C．a common book
D．a car
54．What is the right order that Endal saved Parton according to the passage？
a．Moving Parton into a recovery place．
b．Running back and forth to a nearby hotel．
c．covering Parton with a blanket．
d．Pushing Parton's phone close for him to reach．
A．abcd
B．bcda
C．acdb
D．acbd
55．Which of the followings do you think can describe Endal best according to the passage？
A．Strange．
B．Unbelievable．
C．Hardworking．
D．Honest．