Basalt, which composes most of the ocean floor, is a dark-gray rock rich in iron and magnesium than most surface rocks.

answer: D. But I am wondering how to correct it? I did not find any error for D.

pls be sensitive to some words, such as:

看到than--找比较级；

看到same--找as；

看到too,very划线--找that；so--that

还有很多，此题richer