$A$ hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in a tissue in which the speed of sound is $1.7 \, km \, s^{-1}$? The operating frequency of the scanner is $4.2 \, MHz$ $(1 \, MHz = 10^{6} \, Hz)$.

• A
  $4 \times 10^{-4} \, m$
• B
  $4.2 \times 10^{-4} \, m$
• C
  $1.7 \times 10^{-4} \, m$
• D
  $2.5 \times 10^{-4} \, m$