單項(xiàng)選擇題靜脈穿刺時(shí),當(dāng)針頭進(jìn)入靜脈后應(yīng)沿靜脈方向前進(jìn)多長距離再固定針頭位置?()

A.3cm
B.2.5cm
C.2cm
D.1cm
E.0.5cm