2. We must know the _____ of other countries, so that they will not think us ill-mannered.

A．customs

B．habits

C．manners

D．hobbies

所谓稀疏矩阵指的是________。

A．非零元素个数较多的矩阵

B．零元素个数占矩阵元素总个数一半的矩阵

C．零元素个数远远多于非零元素个数且分布没有规律的矩阵

D．包含有零元素的矩阵