请问一下有经验的学哥学姐们，我现在是专科生，我...

求曲线y=x2与该曲线在x=a(a>0)处的切线与x轴所围的平面图形的面积．

The destruction of habitats(栖息地)all over the world is the primary reason species are?becoming extinct(灭绝)or endangered．Houses,highways,dams,industrial buildings,and?ever-spreading farms now dominate?21?formerly occupied by forests,deserts,and wetlands．22?the beginning of European settlement in America,23,
over 65,000,000 acres of?wetlands have been drained．One million acres alone vanished
24?1985 and 1995．
Habitat destruction can be?25?or it can be subtle,occurring over a
26?period of time?without being noticed．27?such as sewage from cities and chemical runoff fromfarms,can change?the?28?and quantity of water in streams and rivers．To?29?living in a delicately balanced?habitat,this disturbance can be as
30?as the clear-cutting of a rainforest．
31?remaining habitats are carved into smaller and smaller pockets or islands,remaining?species are forced to exist in these?32?areas,which causes further habitat?33?These?species become less adaptable to environmental?34；in fact,they become?35?endangered．Scientists believe that when a habitat is cut by 90%,one-half of its plants,animals and insects will?become extinct．
第28题的答案是（ ）

A.Amount
B.purity
C.nature
D.quality