Which of the following is true to implement the Stock Exchanges and Stocks classes and the association between them in the preceding UML class diagram in a database?

1-The primary key of Stocks is a foreign key in Stock Exchanges.

2-The primary key of Stock Exchanges is a foreign key in Stocks.

3-The association between Stocks and Stock Exchanges is implemented as a linking table.

4-Both "The primary key of Stocks is a foreign key in Stock Exchanges" and "The primary key of Stock Exchanges is a foreign key in Stocks" are true.

5-None of these is true.