The bus was due to __________ at 5 p. m.，but it arrived one hour late due to __________ by the heavy fog.

A．arrive; be held up

B．arrive; being held up

C．arriving; be held up

D．having arrived; having been held up

采用溴量法测定司可巴比妥钠的含量时必须具备的条件是（）

A.加入定量过量溴滴定液

B.在盐酸介质中进行滴定

C.用硫代硫酸钠滴定液滴定

D.用淀粉指示剂指示终点

E.加入过量的碘化钾溶液