$9.$ You are standing between two stereo speakers that are emitting sound of wavelength $10\mathrm{.}0$ cm in step with each other. They are very far apart compared with that wavelength. If you start in the middle and walk a distance $\backslash ($ x $\backslash )$ directly toward one speaker, you observe that the sound from these speakers first cancels when $\backslash ($ x $\backslash )$ is equal to

A$.20\mathrm{.}0$ cm $.$

B$.10\mathrm{.}0$ cm $.$

C$.5\mathrm{.}0$ cm $.$

D$.2\mathrm{.}5$ cm $.$