A string vibrates with a natural fundamental frequency of 220 Hz, with a wavelength of 2m. Besides 220 Hz, which of the following are "resonant frequencies" which you might also faintly detect?

a) 110 Hz

b) 330 Hz

c) 440 Hz

BLUE: a only

YELLOW: b only

GREEN: c only

PURPLE: a and c

PINK: all three