1828年，德国化学家维勒冲破生命力学说的束缚，在实验室里将无机物氰酸铵(NH₄CNO)溶液蒸发，得到了有机物尿素 [CO(NH₂)₂]。他的这一功绩，恩格斯曾给予了很高的评价。 下列有关说法中不正确的是  [ ]

A．氰酸铵属于铵盐

B．氰酸铵与尿素化学性质不同

C．氰酸铵转化为尿素的过程属于物理变化

D．有机物不一定要从生命体内获得

The most popular organic gem is the pearl. A pearl is the response of a marine mollusk to the presence of an irritating impurity accidentally introduced into its body; a cultured pearl is the result of the intentional insertion of a mother-of-pearl bead into a live mollusk. Whether introduced accidentally or intentionally, the pearl-making process is the same: the mollusk coats the irritant with a substance called nacre. Nacre is composed chiefly of calcium carbonate. Because very few natural pearls are now on the market, most pearls used in fine jewelry are cultured. These include "Biwa" pearls and most other freshwater pearls. Cultured pearls are not easily distinguished from natural pearls except by an expert.

The difference between a pearl and a cultured pearl is the nature of the________.

A．color

B．introduction of the irritating impurity

C．coating material

D．irritating impurity