Moderate# Freezing a Sine Wave with Strobe Light

WAVES-XVAJKJ

A stroboscope illuminates a continuous sine wave of wavelength $\lambda$ m traveling on a long string. When the strobe is flashing at a frequency of $F$ Hz the wave on the string no longer seems to be moving--the fixed sine shape of the wave appears stationary. What expression below correctly predicts the possible value or values of the wave speed, $v$, on the string?