Which two statements are true about substitution variables?

Which two statements are true about substitution variables? (Choose two.)
A. A substitution variable can be used with any clause in a SELECT statement
B. A substitution variable used to prompt for a column name must be enclosed in a single quotation marks
C. A substitution variable prefixed with & always prompts only once for a value in a session
D. A substitution variable can be used only in a SELECT statement
E. A substitution variable used to prompt for a column name must be enclosed in double quotation marks
F. A substitution variable prefixed with && prompts only once for a value in a session unless it is set to undefined in the session

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