Which of the following statements about entity-relationship diagrams (ERDs) are correct? Select all that apply. Explain if incorrect. 

a. An entity usually represents persons, places, things, or events about which we want to store data. 

b. A foreign key is an attribute that uniquely identifies each instance of the entity. 

c. A composite key is a key that consists of more than one attribute. 

d. A relationship between entities represents certain business facts or rules.