Which two statements are true about substitution variables?

A substitution variable prefixed with & always prompts only once for a value in a session.

A substitution variable prefixed with to prompts only once for a value in a session unless it is set to undefined in the session

A substitution variable can be used only in a select statement.

A substitution variable used to prompt for a column name must be enclosed in single quotation marks.

A substitution variable can be used with any clause in a select statement.

A substitution variable used to prompt for a column name must be enclosed in double quotation marks.