7. A diagnostic ultrasound-echo is reflected from the moving away blood and returns with a frequency 500Hz lower than its original 2.0MHz. The speed (in unit of m/s) of the blood is about? The ultrasound speed is the same as sound speed which is~1600 m/s in blood.(A) 0.2 (B) 0.02 (C) 0.002 (D) 0.0002 (E) 0.00002