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41. 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. The wavelength of sound in the tissue is close to

$$\begin{gathered} \left(a\right) 4 \times {10}^{-4} m \\ \left(b\right) 8 \times {10}^{-4} m \\ \left(c\right) 4 \times {10}^{-3} m \\ \left(d\right) 8 \times {10}^{-3} m \end{gathered}$$