单项选择题

Questions 10-11 are based on the following passage.
According to the theory of plate tectonics,
the lithosphere (earth’s relatively hard and
solid outer layer consisting of the crust and
Line part of the underlying mantle) is divided
(5) into a few dozen plates that vary in size and
shape; in general, these plates move in rela-
tion to one another. They move away from
one another at a mid-ocean ridge, a long
chain of sub-oceanic mountains that forms a
(10) boundary between plates. At a mid-ocean
ridge, new lithospheric material in the form
of hot magma pushes up from the earth’s
interior. The injection of this new lithos-
pheric material from below causes the phe-
(15) nomenon known as sea-floor spreading.
Given that the earth is not expanding in
size to any appreciable degree, how can
"new" lithosphere be created at a mid-ocean
ridge For new lithosphere to come into
(20) being in one region, an equal amount of
lithospheric material must be destroyed
somewhere else. This destruction takes place
at a boundary between plates called a sub-
duction zone. At a subduction zone, one
(25) plate is pushed down under another into the
red-hot mantle, where over a span of mil-
lions of years it is absorbed into the mantle. According to the passage, a mid-ocean ridge differs from a subduction zone in that