I'll go to the library as soon as I finish what I _____.[ ]

A. was doing

B. am doing

C. have done

D. had been doing

道床支承块由厂家在工地按工厂化标准预制，混凝土配合比由()，钢筋、水泥、骨料、水、外加剂等符合设计要求。

A.监理工程师确定，混凝土采用强制式搅拌机拌和 

B.监理工程师确定，混凝土采用自落式搅拌机拌和 

C.工厂试验确定，混凝土采用强制式搅拌机拌和 

D.工厂试验确定，混凝土采用自落式搅拌机拌和