If x(n) is a real sequence, then what is the value of XI( )?

• A. ( sum_{n=- }^ x(n)sin u2061( n) )
• B. ( sum_{n=- }^ x(n)sin u2061( n) )
• C. ( sum_{n=- }^ x(n)cos u2061( n) )
• D. ( sum_{n=- }^ x(n)cos u2061( n) )