_____________ many times, the problems were settled at last.

A．Having discussed

B．Being discussed

C．To be discussed

D．Discussed

锅炉制造中，使用摩擦焊较典型的接头是（）。

A.集箱端盖对接

B.汽包筒体环缝

C.蛇形管对接

D.烟管角接头