An elastic string is fastened to an oscillator at one end and held by a clamp at the other end, with the string kept horizontal and under tension. The oscillator is then made to move vertically. Starting from zero, the oscillation frequency is increased gradually until a stationary wave forms in the string. The frequency is then raised again to frequency $f$ when a second stationary wave forms in the string. The frequency is increased once more. At which frequency does a third stationary wave occur?
- A$1.2f$
- B$1.5f$
- C$2.0f$
- D$3.0f$