In the Late 20th Century Most Metal Prices Were Very

In the late 20th century most metal prices were very low but metal prices increased dramatically in the early 21st century.Simultaneously,the early 21st century saw extensive "brown fields exploration" in which companies went to old mining areas and extracted old mine wastes or reopened old mine workings.What is the primary explanation for this activity?

A) The old miners were wasteful and left large amounts of ore in the ground.
B) The increase of metal prices made mineral resources that were previously uneconomic into ores that could potentially be extracted profitably.
C) Environmental regulations make it impossible to explore anywhere but old mining areas,so this was the only place the companies could look for deposits.
D) The companies were only interested acquiring properties through a sleazy land grab,and had no intention of doing anything with the deposits assuming no one cared about brown fields.