Seamless mild steel is used for steam (also copper for moderate pressure and temperature), h-pair, feed discharge and all oil fuel pressure pipingsAccording to the above sentence, which of the following statements is not true?（）

A. Both seamless mild steel and copper may be used for middle pressure steam
B. "hpair" means high pressure air
C. The feed piping of the boilers can be made of seamless mild steel
D. All oil pipings are made of mild steel