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.
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
formerly occupied by forests, deserts, and wetlands.
the beginning of European settlement in America,
, over 65,000,000 acres of wetlands have been drained. One million acres alone vanished
1985 and 1995.
Habitat destruction can be
or it can be subtle, occurring over a
period of time without being noticed.
such as sewage from cities and chemical runoff from farms, can change the
and quantity of water in streams and rivers. To
living in a delicately balanced habitat, this disturbance can be as
as the clear-cutting of a rainforest.
remaining habitats are carved into smaller and smaller pockets or islands, remaining species are forced to exist in these
areas, which causes further habitat
. These species become less adaptable to environmental
;in fact, they become
endangered. Scientists believe that when a habitat is cut by 90%, one-half of its plants, animals and insects will become extinct.