使用SQL语句增加字段的有效性规则，是为了能保证数据的

A．实体完整性

B．表完整性

C．参照完整性

D．域完整性

He made another wonderful discovery, ______ of great importance to science.

A．which I think it is

B．which I think is

C．which I think

D．I think which is