在列管式换热器中，冷流体的进出口温度为20℃和80℃，热流体的温度由150℃降为90℃，则并流时的平均温差是        。

A、<span style="font-size:14px;font-family: &#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">46.8</span><span style="font-size:14px;font-family:&#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">度</span>

B、<span style="font-size:14px;font-family: &#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">40</span><span style="font-size:14px;font-family:&#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">度</span>

C、<span style="font-size:14px;font-family: &#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">50</span><span style="font-size:14px;font-family:&#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">度</span>

D、<span style="font-size:14px;font-family: &#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">60</span><span style="font-size:14px;font-family:&#39;微软雅黑&#39;,&#39;sans-serif&#39;;color:#222222">度</span>