Which of the following is true to implement the Stock Exchanges and Stocks classes and the association

Question:

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?
a. The primary key of Stocks is a foreign key in Stock Exchanges.
b. The primary key of Stock Exchanges is a foreign key in Stocks.
c. The association between Stocks and Stock Exchanges is implemented as a linking table.
d. Both a and b are true.
e. None of these is true.