Which statement is true about an External ID field? (Choose two.)

Which statement is true about an External ID field? (Choose two.)
A . The field contains unique record identifiers from a system outside of Salesforce.
B . The field must be unique since duplicate are not allowed within Salesforce.
C . The field must contain at least one number and at least one letter.
D . The field can be unique based on case-sensitive or case-insensitive values.

Answer: AD