Which of these fits gap 19?

Directions: Complete this passage by selecting the most suitable option for the corresponding gap.
History reveals that copper was used by man as early as the Stone Age. By 4000 BC, mining of copper______ (17) _______.
_____ (18) ______, man came to know that bronze, an alloy of copper, zinc and tin is harder than copper.
As time ____ (19) ______, copper came to be one of the most important metals in the world, used extensively in electrical and other industries.
______ (20) ______ of the world’s copper comes from Canada, USA, Chile, Zambia and Russia.