19．How much do you know about the bird world？
Birds use three gaits：walking，running and hopping．
In walking，the left and the right feet move in turn（轮流）．Each foot is on the ground for more than half the time，so there are more times when both feet are on the ground．In running，each foot is on the ground for less than half the time，and there are more times when both feet are off the ground．In hopping，the feet move almost at the same time．This means that for much of the time both feet are off the ground．
Middle-sized and large birds walk at low speed and either run or hop at higher speed．Small birds，such as sparrows and swallows，usually do not walk，because walking may be too slow for them．It seems that the more time a bird spends in trees，the more likely it is to hop．Perhaps there is wide space in a tree between one thin branch（树枝） and the next，so walking is not a good choice，while on each small branch there is enough room to set both feet down side by side，so hopping seems to make sense．
根据短文内容，选择最佳答案，并在答题卡上将选定的答案标号涂黑．
66．How is the bird moving when both feet are on the ground for more than half the time？C
A．By flying．
B．By running．
C．By walking．
D．By hopping．
67．Which of the following shows a bird is hopping？B
68．Why do the birds in trees usually move by hopping？A
A．Space between the two branches is wide．
B．The birds in trees are small in size．
C．There is little room to set the birds'feet on．
D．Branches are high in the air off the ground．
69．Which of the following usually moves by hopping？D
A．A duck．
B．A chicken．
C．A snake．
D．A sparrow．
70．What does the underlined word"gaits"mean？C
A．Directions of flying．
B．Speeds of moving．
C．Ways of moving．
D．Times of staying in trees．
