I ____ in the street when I met an old friend of mine the other day.

A．walked

B．was walking

C．has walked

D．had walked

下述哪种熔化合金热源不能用于熔化高熔合金的热源？（）

A.高频感应热源

B.中频感应热源

C.汽油吹管火焰热源

D.弧热源

E.铂电阻丝热源