[問題PP Test1-reading-第22篇 Q234

[kool]

In essence, the formation of a limestone cave is
not particularly complicated, but it may take millions
of years to first produce the basic cavity and then to
decorate it with cavern formations. Caves can be said
to have 3 growth phases: excavation, decoration, and
dormancy.
Beds, or strata, of limestone or marble
(metamorphosed limestone) are commonly faulted,
cracked, and fractured by movements of the Earth's
surface and certain parts of a strata may be much
softer than others. It is along these planes of
weakness that caverns develop. Water excavates
or hollows out the caverns.
Rain, snowmelt, and other groundwaters
containing carbon dioxide seep or flow downward
along cracks, joints, fault planes, and fissures. This
water actually constitutes a weak form of carbonic
acid and slowly dissolves the limestone. Gradually
this dissolving action enlarges the cracks, joints, and
crevices into passageways, rooms, or huge halls.
These barren, dissolved areas are called solution
channels.
The rate of erosion for a cave is relative,
depending on the amount of water involved, the
length of time, the rate of flow, and the resistance of
the particular limestone. Some caves have apparently
been created by an underground stream or river
flowing along the line of least resistance deep in the
Earth. After solution channels have been excavated,
rooms may fill up with very slow moving water.
This water acts on the walls of the room, gradually
expanding the chamber by dissolving the walls and
causing the ceiling to collapse. The ceiling rubble
is then attacked by the water, which in turn dissolves
it and carries it away. This is somewhat of an
oversimplification, but gigantic rooms, such as those
in Carlsbad Caverns in the western United States,
have been created by this process.
Eventually the prime dissolving waters leave
the caverns. This may be due to geological uplift
of the region, causing the water to drain away, or
perhaps the original source is diminished, terminated,
or diverted. Then air fills the chambers---most
accessible caverns have an abundance of fresh air
in them. Air and water are essential to the second
phase of cavern development, decoration.



234. The word constitutes in the passage
is closest in meaning to

A.finds out
B.consists of
C.causes
D.prevents



ANS: B

可是consist of(由...組成),而constitute(組成,構成)
ex: Water consists of hydrogen and oxygen.
Hydrogen and oxygen constitute water.

意思不是正好有點相反的味道ㄇ



請幫忙解答...感溫..^^