3．The discovery of an ancient giant panda skull has confirmed its bamboo diet dates back more than 2 million years and may have played a key part in its survival．
A Chinese-US research team reports its results today following studies on a fossil skull found in south China's Cuangxi Zhuang Autonomous Region in 2001．
The six fossils unearthed in Jinyin Cave are dated between 2.4 and 2 million years ago，according to the report in Proceedings of the National Academy of Sciences，an influential US journal．
Jin Changzhu，of the chinese Academy of Sciences （CAS） and lead author of the paper，said the smaller fossil skull indicates the giant pandas were about a third smaller than today's pandas．
Researchers knew the panda reached its maximum size about 500，000 years ago，when it peaked，and then gradually became smaller．
Jin，a paleontologist （古生物學者） at the Institute of Vertebrate paleontology and Paleoanthropology （古人類學） attached to the CAS，said the size variation was a basic rule of evolution．
"A species tends to grow bigger when it reaches the peak of its population，but becomes smaller when numbers decline，"he said．
The dental remains of the skull，which is the oldest giant panda skull ever found，are similar to today's pandas，indicating the type of teeth that could munch （津津有味地嚼）mountains of bamboo．A panda can eat up to 40kg of bamboo per day．
Paleoanthropologist Russell Ciochon，the US co-author at the University of lowa，said the panda's focus on bamboo could have helped it survive all these years．
"Once an animal begins to rely on a common and stable food source，such as bamboo，it tends to evolve a larger body size，"he said．"As individuals of the evolving species grow bigger，they have a better chance not to be eaten by predators （肉食動物） due to their larger body size．"
33．According to the research of the CAS，there were most pandas in the worldC．
A.2 million years ago
B．between 2.4 and 2 million years ago
C.500，000 years ago
D．Nowadays
34．The underlined word"variation"in para 6 meansB．
A．fall B．change C．increase D．decrease
35．From the passage，we can learnB．
A．pandas began to eat bamboo 2 million years ago
B．a species tends to grow smaller when numbers decline
C．the giant pandas were about a third the size of today's pandas
D．pandas'bamboo has played an important role in its development．
分析 古代大熊貓頭骨的發(fā)現(xiàn)證實了大熊貓吃竹子飲食習慣可追溯到200萬多年，這可能是影響其生存的一個關鍵部分．
解答 33．C．事實細節(jié)題．根據(jù)短文Researchers knew the panda reached its maximum size about 500，000 years ago，when it peaked，and then gradually became smaller．可知500，000年以前的大熊貓數(shù)量是最多的，故選C．
34．B．詞義猜測題．根據(jù)下文"A species tends to grow bigger when it reaches the peak of its population，but becomes smaller when numbers decline，"he said．可知，一個物種的數(shù)量達到頂峰，它們的體型會變大，數(shù)量減少，體積會變小，所以是體型的改變，故選B．
35．B．事實細節(jié)題．根據(jù)短文"A species tends to grow bigger when it reaches the peak of its population，but becomes smaller when numbers decline，"he said．可知當一個物種數(shù)量減少時，這個物種的生物的體型會變小，故選B．
點評 考察學生的細節(jié)理解和推理判斷能力，做細節(jié)理解題時一定要找到文章中的原句，和題干進行比較，再做出正確的選擇．在做推理判斷題不要以個人的主觀想象代替文章的事實，要根據(jù)文章事實進行合乎邏輯的推理判斷．
