Flying squirrels are a family of 44 species of squirrels．The term "flying" is somewhat of an incorrect name．
Flying squirrels are not capable of powered flight like birds or bats; instead, they glide （=move smoothly and quietly） between trees．They are capable of obtaining lift within the course of these flights, with flights recorded to 90 meters （295 ft）．The direction and speed of the animal in midair is varied by changing the positions of its two arms and legs, largely controlled by small soft wrist bones． This changes the tightness of the paganism（翼膜）,a very thin furry parachute-like skin that stretches from wrist to ankle．It has a hairy tail that keeps its flight steady and unchanging．The tail acts as an assistant airfoil （螺旋桨），working as an air brake before landing on a tree trunk．
Flying squirrels, are gliding mammals （哺乳动物）. A few mammals can glide through the trees, but they do not actually fly （like birds and bats）．They have a very soft and thin skin on either side of
their body．
The Siberian Flying Squirrel ranges from the Baltic Sea in the west to the Pacific Ocean in the east．Before the 21st century, the evolutionary （进化） history of the flying squirrel was frequently argued about．This debate became clearer and clearer as a result of two recent scientific studies．These studies found support that flying squirrels originated 18-20 million years ago have a twin relationship with tree squirrels．
69．How does the flying squirrel control its flying speed?
A．By moving its tail．
B．By changing the positions of its arms and legs．
C．By blowing the air．
D．By moving its soft thin skin on either side of the body．
70．_______ can help the flying squirrel stop gliding and land．
A．A tree trunk B．Its bones in the body
C．Its tail D．Its wrists
71．According to this article, all the following statements are true except that _______．
A．flying squirrels and birds are of a kind because both of them can fly
B．millions of years ago, flying squirrels were sisters and brothers of tree squirrels
C．flying squirrels can glide instead of flying
D．flying squirrels give birth to babies rather than lay eggs
72．Which category does the text most probably belong to?
A．Agriculture and Food． B．Health and Medicine．
C．Plants and Animals． D．Science and Technology．