单项选择题

(This passage was written prior to 1950)
The coastlines on the two sides of the Atlantic
Ocean present a notable parallelism: the eastern-
most region of Brazil, in Pernambuco, has a con-
Line vexity that corresponds almost perfectly with the
(5) concavity of the African Gulf of Guinea, while
the contours of the African coastline between Rio
de Oro and Liberia would, by the same approxi-
mation, match those of the Caribbean Sea.
Similar correspondences are also observed in
(10) many other regions of the Earth. This observation
began to awaken scientific interest about sixty
years ago, when Alfred Wegener, a professor at
the University of Hamburg, used it as a basis for
formulating a revolutionary theory in geological
(15) science. According to Wegener, there was origi-
nally only one continent or land mass, which he
called Pangea. Inasmuch as continental masses
are lighter than the base on which they rest, he
reasoned, they must float on the substratum of
(20) igneous rock, known as sima, as ice floes float on
the sea. Then why, he asked, might continents not
be subject to drifting The rotation of the globe
and other forces, he thought, had caused the
cracking and, finally, the breaking apart of the
(25) original Pangea, along an extensive line repre-
sented today by the longitudinal submerged
mountain range in the center of the Atlantic.
While Africa seems to have remained static, the
Americas apparently drifted toward the west until
(30) they reached their present position after more than
100 million years. Although the phenomenon
seems fantastic, accustomed as we are to the con-
cept of the rigidity and immobility of the conti-
nents, on the basis of the distance that separates
(35) them it is possible to calculate that the continental
drift would have been no greater than two inches
per year.

The primary purpose of the passage is to（）

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