A hospital uses an ultrasonic scanner to locate tumours in a tissue. The operating frequency of the scanner is 4.2 MHz. The speed of sound in a tissue is 1.7 km-s^–1. The wavelength of sound in the tissue is close to

• A

  $4\times {10}^{-4}\mathrm{m}$

• B

  $8\times {10}^{-3}\mathrm{m}$

• C

  $4\times {10}^{-3}\mathrm{m}$

• D

  $8\times {10}^{-4}\mathrm{m}$