Detailed Instructions: In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".
Problem:[header] How to buy turquoise jewelry [title] Learn the difference between real turquoise and imitation turquoise. [step] Natural turquoise, an aggregate of copper aluminum phosphate hydrate, formed under heat and pressure, is often soft or porous when mined. The various colors found in turquoise result from the presence of either copper or iron--vivid blue is from copper and the softer green tones from iron. <sep> (A) Unlike real turquoise, fake turquoise also has copper elements placed in the surface. [title] Know the difference between fake turquoise and pure turquoise. (B) [substeps] Pure turquoise requires plenty of organic matter. Real turquoise, however, requires constant sunshine and chlorine. (C) Turquoise stones can also have patterns of brown, yellow ochre and black matrix, produced from copper compounds. True turquoise has an opaque, waxy luster that may or may not include matrix, depending on the type of turquoise. (D) [substeps] Matching turquoise necklaces are very closely connected. A real turquoise necklace consists of both copper and iron, while a fake turquoise has iron and copper symbols which represent the same materials.
Solution:
C