The "Karat" marking on jewelry tells you w

问题：

The "Karat" marking on jewelry tells you what proportion of gold is mixed with other metals. If 14 parts of gold are mixed with 10 parts of base metal, the combination is called 14-Karat (14K) gold. The higher the Karat rating, the higher the proportion of gold in the object. The lowest Karat gold that can be marketed in the United States is 10-Karat gold. Jewelry does not have to be marked with its Karat quality, but most of it is. If there is a Karat quality mark, next to it must be the U.S. registered trademark of the person or company that will stand behind the mark, as required by the National Gold and Silver Stamping Act.

Gold which is 10 Karats in proportion ________in the U.S.

A．represents the highest grade of gold

B．cannot be sold

C．never carries a Karat quality mark

D．represents the lowest-grade gold marketable