低相对分子质量肝素的抗因子Xa活性较佳的维持范围是

A．0.5～4.0个抗因子Xa/ml

B．1.5～4.0个抗因子Xa/ml

C．2.5～6.0个抗因子Xa/ml

D．2.5～7.0个抗因子Xa/ml

E．2.5～8.0个抗因子Xa/ml