32．_____from lightweight materials，the robots are about the size of a human arm or

smaller．

A．To build

B．Being built

C．Having built

D．Built

按锅炉的工作原理来分，热水锅炉可分（）和强制循环热水锅炉。

A、水管锅炉

B、自然循环热水锅炉

C、火管锅炉

D、水火管锅炉