How do you add a foreign key to an existing table created by another module?

How do you add a foreign key to an existing table created by another module?
A . Create etc/db_schema.xml file with the table node and constraint child node
B . Run the command bin/magento setup:db-schema:upgrade <table> <constraint declaration>
C . This can only be done with raw SQL in a Schema Patch file
D . Create the etc/db_constraints.xml file and specify foreign key there in the constraint node

Answer: A

Explanation:

https://magento.stackexchange.com/questions/192317/magento-2-how-to-add-foreign-key-in-update-schema