单选19．有可能成为被控变量的是(  )

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

B、<span style="font-size:16px;font-family:宋体">加热器</span><span style="font-size: 16px;font-family:&#39;Times New Roman&#39;,&#39;serif&#39;">&nbsp; &nbsp;</span>

C、<span style="font-size:16px;font-family:宋体">水箱</span><span style="font-size:16px;font-family:&#39;Times New Roman&#39;,&#39;serif&#39;">&nbsp; &nbsp;&nbsp;</span>

D、<span style="font-size:16px;font-family:宋体">液罐</span>