18. An ultrasound pulse is travel- ing through muscle, which has Material Acoustic Impedance (rayls) an acoustic impedance of 1.75 x Air 4.0 x 10 10 rayls, when it encounters an ob- Lung 1.80 x 10 stacle. You observe that 40% of the Fat 1.35 x 10 incident intensity is reflected back. Blood 1.70 x 10 What can you conclude that the Bone 7.8 x 10The two sources of sound, A and B. produce waves in phase with one another so that a peak leaves each source at the same moment. The wavelength is 1 = 2.00m and the distance between A and B is d = 2.50 m. At which of the follow distances x to the right of A will the waves from the two sources interfere destructively. B d = 2.50 m A. x = 0.50 m B. x = 0.67 m C. x = 1.00 m D. x = 1.25 m E. x = 1.75 m