版主和G友们帮忙一下：一篇短阅读求解，关于地幔的。

Y.LuffyUID: 2996786

Geologists have long known that the
　　Earth's mantle is heterogeneous, but its
　　spatial arrangement remains unresolved—
　　is the mantle essentially layered or
(5) irregularly 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
(10) 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.
(15) 　Some geologists, however, on the basis
　　of observations concerning mantle
　　xenoliths, argue that the mantle is
　　not layered, but that heterogeneity is
　　created by fluids rich in "incompatible
(20) elements" (elements tending toward
　　liquid rather than solid state)
　　percolating upward and transforming
　　portions of the upper mantle
　　irregularly, according to the vagaries
(25) of the fluids' pathways. We believe,
　　perhaps unimaginatively, that this
　　debate can be resolved through further
　　study, and that the underexplored
　　midocean ridge system is the key.

19. It can be inferred from the passage that thesupporters of the “layered-mantle” theory believe which of thefollowing?
I. The volcanicrocks on oceanic islands are composed of material derived from thelower part of the mantle.
II. The materials of whichvolcanic rocks on oceanic islands and midocean ridges arecomposed are typical of the layersfrom which they are thought tooriginate.
III. The differences in composition betweenvolcanic rocks on oceanic islands and the midocean ridges are aresult of different concentrations of incompatible elements.×
(A) I only
(B) III only
(C) I and II only
(D) II and III only
(E) I, II, and III


答案中的I我理解了，II该如何理解？我把原文的句子狠读了几次还是没觉得有提到这个啊。

苏梳眠UID: 2717864

核心词是originate~
回到第十行左右，可以看到oceanic islands上的岩石材料来自lower mantle，而midocean ridge system上的岩石材料来自upper mantle~
检出核心词arising from 和source~

银杏叶子UID: 2925999

II选项的意思是海岛上火山岩和海脊的构成物质是由他们各自起源的地层物质构成的。即火山岩是由下层地幔的物质喷发到上层形成的，而海脊的构成物质是本来就在地幔上层的物质。这个跟原文那个well-established fact讲的是一件事。
个人见解，仅供参考。