What are two reasons an existing custom field cannot be marked as External ID?

Salesforce Mobile-Solutions-Architecture-Designer Question Answer

What are two reasons an existing custom field cannot be marked as External ID? Choose 2 answers

Maximum number of External IDs allowed on an objecthas been reached,

Maximum number of fields of an object has been reached.

Maximum number of skinny tables has been reached.

Maximum number of indexes allowed on an object has been reached.

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