When he was a boy, he used to go there and watch _____.

A．to repair bicycles

B．bicycles to be repaired

C．bicycles being repaired

D．repairing bicycles

锅炉蒸汽吹管的范围包括（）、（）系统，减温水管道系统，锅炉过热器、再热器系统的（）和对空排气管道等。