Several of the chemical elements were known in prehistoric【C1】______ . One of the great【C2】______ forward in civilization came when bronze, an alloy,【C3】______ stone as the main material for tools and weapons. The iron replaced bronze. The first elements【C4】______ were those that were found free in nature or could be readily【C5】______ from their ores at relatively low temperatures. During the Middle Ages, chemistry was mostly practised by the alchemists,【C6】______ tried to turn base metals into gold, and who were interested in discovering a way of living for【C7】______ . But they discovered many important【C8】______ about the metals and some of the most easily attainable compounds. Even before the Middle Ages, people knew how to【C9】______ mercury, tin, and iron from their ores. By the time of Paracelsus, chemistry was developed largely to the effort of【C10】______ medicines.
【C1】

A. times
B. schedules
C. plans
D. timing