Magnesium is another mineral we now obtain by collecting huge volumes of ocean water and t

reating it with chemicals. Although originally it was derived only from brines or from the treatment of such magnesium-containing rocks as dolomite, of which whole mountain ranges are composed. In a cubic mile of sea water there are about four million tons of magnesium. Since the direct extraction method was developed about 1941, production has increased enormously. It was magnesium from sea that made possible the wartime growth of the aviation industry, for every, air plane made in the United States (and in most other countries as well) contains about haft a ton of magnesium metal. And it has innumerable uses in other industries where a lightweight metal is desired, besides its long-standing utility as an insulating material, and its use in printing inks, medicines, and toothpaste.

What is the main topic of this passage？

A．Use of sea water.

B．Treatment of sea water.

C．Chemical properties of magnesium.

D．Derivation and users of magnesium.