Which of the following is not a factor which can interfere with the pulse of sound and cause a variation the depth reading?（）

A.salinity

B.temperature

C.the hydrostatic pressure

D.Vessel’s speed

关于肝素的说法中，正确的是

A．肝素带正电荷
B．肝素易通过生物膜
C．口服后立即生效
D．在体内外均有强大的抗凝作用
E．半衰期较长