Two speakers are driven by the same oscillator at frequency 420 Hz. One speaker is mounted on the ceiling, and the other on the wall at head level, a horizontal distance of 2 m away. When an observer stands directly underneath the ceiling speaker, she hears a sound minimum. Which of the following is a possible height for the speaker above her head?

a) 1.36 m b) 1.60 m c) 1.29 m d) 1.43 m e) 1.53 m