Final answer:
Open pit mining is done mainly because the ore deposits are either on the surface or close to the surface, which makes them economically viable to be accessed and mined. Techniques have evolved from shallow digging during the early gold rushes to more modern methods as easily accessed deposits became exhausted. Nowadays, large-scale open pit mining is common for various minerals, such as gold and salt. The correct answer is option a.
Step-by-step explanation:
Open pit mining is done primarily because ore deposits are either exposed or close to the land surface. This method is typically employed when the minerals being sought are situated in near-surface environments and can be mined economically by removing the ground cover and excavating or blasting the mineral deposits. Traditional gold prospecting was replaced by open pit mining and other large-scale operations due to the depletion of easily accessible minerals, necessitating more extensive and technically advanced methods to extract remaining mineral deposits.