阅读求解？

maliana

Geologists have long known that the Earth‘s mantle is heterogeneous, but its spatial arrangement remains unresolved—is the mantle essentially layered or irregu-larly heterogeneous? The best evidence for the layered-mantle thesis is the well-established fact that volcanic rocks found on oceanic islands, islands believed to result from mantle plumes arising from the lower mantle, are composed of material fundamentally different from that of the midocean ridge system, whose source, most geologists contend, is the upper mantle. Some geologists, however, on the basis of obser-vations concerning mantle xenoliths, argue that the mantle is not layered, but that heterogeneity is created by fluids rich in “incompatible elements” (elements
tending toward liquid rather than solid state) percolating upward and transforming portions of the upper mantle irregularly, according to the vagaries of the fluids‘ pathways. We believe, perhaps unimaginatively, that this debate can be resolved
through further study, and that the under explored midocean ridge system is the key.
(157 words)

For the following question, consider each of the choices separately and select all that apply
11. It can be inferred from the passage that the sup-
porters of the ―layered-mantle‖ theory believe which
of the following?
□A The volcanic rocks on oceanic islands are composed of material derived from the lower part of the mantle.
□B The materials of which volcanic rocks on oceanic islands and midocean ridges are composed are typical of the layers from
which they are thought to originate.
□C The differences in composition between volcanic rocks on oceanic islands and the midocean ridges are a result of different
concentrations of incompatible elements.
答案为AB
疑问是A为什么正确
难句中这样解释答案来源的那句话：对于地幔分层论点来说， 最好的证据乃这样一个确认事实即在那些海洋岛屿 —— 这些岛屿据信是源于由下层地幔升上来的地幔柱状溶岩流—— 上发现的火山岩石，是由与海洋中部脊系统物质根本不同构成，而这一海洋中部山脊系统的成因，大多数地质学家论辩道为上幔。
只觉得文中讲到火山岩构成不同于山脊成因？

maliana

疑问求解 ？？