Directions: For each blank in the following passage, there are four choices marked A, B, C and D. Choose the one that is most suitable and mark your answer by blackening the corresponding letter on the Answer Sheet.

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 from farms 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.