问题
单项选择题 A1/A2型题
固定桥承受力时，固位体产生（）
A.负重反应
B.屈矩反应
C.外张力
D.屈应力
E.以上均不是
答案
参考答案：B
搜题请搜索小程序“爱下问搜题”
固定桥承受力时，固位体产生（）
A.负重反应
B.屈矩反应
C.外张力
D.屈应力
E.以上均不是
参考答案：B
| 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 free 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. |
Nacre is a substance that is ______.
A．mechanically manufactured
B．the result of laboratory testing
C．organically secreted by the mollusk
D．present in the chemical composition of freshwater pounds