【B】Most of Africa is a series of stable, ancient plateau surfaces, low in the north and west andhigher in the south and east. The plateau is composed mainly of metamorphic rock that has beenoverlaid in places by sedimentary rock. The escarpment of the plateau is often in close proximity tothe coast, thus leaving the continent with a generally narrow coastal plain; in addition, the escarpmentforms barriers of falls and rapids in the lower courses of rivers that impede their use as transportationroutes into the interior. The entire African continent is surrounded by a narrow continental shelf. Thelowest point on the continent is 155 meters below sea level in Lake Assal in Djibouti; the highest pointis Mt. Uhuru, a peak of Kilimanjaro in Tanzania, at 5,895 meters in elevation.
        The continent’s largest river is the Nile, which is also the world’s longest river. The largest lakeis Victoria, which is the world’s second largest freshwater lake. There are a number of other rivers andlakes throughout the continent. These lakes and major rivers form an important inland transportationsystem.
        Geologists have long noted the excellent fit between the coast of Africa at the Gulf of Guinea andthe Brazilian coast of South America, and they have evidence that Africa formed the center of a largeancestral supercontinent known as Pangaea. Pangaea began to break apart in the Jurassic period toform Gondwanaland, which included Africa, the other southern continents, and India. South Americawas separated from Africa 76 million years ago, when the floor of the South Atlantic Ocean wasopened up by seafloor spreading. There is also evidence of one-time connections between NorthwestAfrica and Eastern North America, North Africa and Europe, Madagascar and India, and SoutheastAfrica and Antarctica.
        76 million years is, of course, a blink of an eye in geological time. The long-term future of Africaas a continent is therefore bleak; although we are accustomed to thinking of Africa as an ancient landand as the birthplace of humanity, it is just as susceptible to the geological forces of a cooling planetas the other continents. Having been formed by these forces, it is unlikely that a continent resemblingAfrica will still exist in 100 million years.
【題組】36.Which of the following is the main topic of the passage?
(A) The diversity of Africa’s people.
(B) The longest river in the world.
(C) The formation of the continents in Africa.
(D) The geography of the African continent.