Knowledge of Keys

Which of the following statements are NOT correct about primary and foreign keys in a database?

1. A table in a database can only have a single primary key.

2. Primary keys might contain NULL values

3. You cannot write a query to drop a foreign key constraint as it is linked to a primary key.

Options

1. Only 3
2. 1 and 3
3. Only 2
4. 2 and 3
5. All are incorrect