氯化钠与金刚石型结构是复式格子还是布拉维格子，各自的基元为何？写出这两种结构的原胞与晶胞基矢，设晶格常数为a。

Divorce is the act by which a valid marriage is dissolved, usually freeing the parties to remarry. In regions in (1) ancient (2) authority still predominates, divorce may be (3) and rare, especially when, as among Roman Catholics and Hindus, the religious (4) views marriage (5) indissoluble. Custom, (6) , may make divorce a simple matter in (7) societies. (8) some Pueblo Indian tribes a woman could divorce her husband (9) leaving his moccasins on the doorstep. The (10) of individual determination and mutual (11) are making divorce (12) acceptable in the (13) parts of the world.

Among premodern societies, the rate of marital stability is difficult to (14) (15) the varying definitions of (16) and divorce. It seems to be broadly true (17) wherever divorce is a legal impossibility the wedding is a well-defined event conducted with (18) formality. The (19) principle does not hold true: elaborate marriage ceremonial is quite compatible with high divorce rates. Many anthropologists agree that divorce is generally more permissible in matrilineal societies (20) in patrilineal ones, in which the procreative and sexual rights of the bride are often symbolically transferred to the husband with the payment of bride-price.

18（）

A.less

B.little

C.normal

D.considerable