17. For a standing wave mode on a string fixed at both ends, adjacent antinodes are separated by a distance of 20 cm . Waves travel on this string at a speed of 1200cm/s. At what frequency is the string vibrated to produce this standing wave?
(A) 120 Hz
(B) 60 Hz
(C) 40 Hz
(D) 30 Hz
(A) 120 Hz
(B) 60 Hz
(C) 40 Hz
(D) 30 Hz