Mrs. White found her husband ________by letters and papers and________ very worried.

A．surrounding ; looked

B．surrounded ; looked

C．surrounding; looking

D．surrounded ; looking

用脉冲多普勒技术除下列哪项外，均可增大检测血流速度最大值（）。

A.降低发射频率

B.增大探测角度

C.减低取样深度

D.移动零位基线

E.提高脉冲重复频率