The correct answer is: B. contemporarily
The word “contemporarily” means “at the same time.” In a blast furnace, many reactions take place at the same time, including the reduction of iron ore to iron, the combustion of coke to carbon monoxide, and the oxidation of limestone to calcium oxide. These reactions are essential to the production of iron.
The other options are incorrect because they do not accurately describe the timing of the reactions that take place in a blast furnace. The word “invariably” means “always,” but the reactions in a blast furnace do not always take place at the same time. The word “separately” means “individually,” but the reactions in a blast furnace do not take place individually. The word “momentarily” means “for a very short time,” but the reactions in a blast furnace take place for a long period of time.