The entity-relationship（E-R）data model is based on a perception of a real world that consists of a clletion of basic objects,called（ ），and of relationships among these objects.An entity is a“thing”or"object"in the real world that is distinguishable from other objects.Entities are described in a database by asetof（ ）:A relationship isan association among several entities.The set of all entities of the same type and the set of all relationships of the ametype are termed an entity sct and relationship set,respectively.The overall logical structure（schema）of a database can be expressed graphically by an E-R diagram,which is built up from the fllowing components:（ ）represent entity set,（ ）represent atributes,ete.In addition to entities and relations,the E-R model represents certain（请作答此空）to which thecontents of a database must conform.The entity-relationship model is widely used in database design.
A.things
B.objects
C.conditions
D.constrains
