变直径管，直径d1=320mm， d2=160mm，流速v1=1.5m/s。v2为         。

A、<span style="font-size:16px;font-family:&#39;Times New Roman&#39;,&#39;serif&#39;">6m</span><span style="font-size:16px;font-family:&#39;Times New Roman&#39;,&#39;serif&#39;">/s &nbsp; &nbsp; &nbsp;&nbsp;</span>

B、<span style="font-family: &#39;Times New Roman&#39;, serif;">4m</span><span style="font-family: &#39;Times New Roman&#39;, serif;">/s &nbsp; &nbsp; &nbsp;</span>

C、<span style="font-family: &#39;Times New Roman&#39;, serif;">3m</span><span style="font-family: &#39;Times New Roman&#39;, serif;">/s &nbsp;</span>

D、<span style="font-family: &#39;Times New Roman&#39;, serif;">9m</span><span style="font-family: &#39;Times New Roman&#39;, serif;">/s</span><span style="font-family: 宋体;">。</span>