Surface type of mining is generally more expensive in terms of cost- per-unit of production 5видоа вопросов к этому предложению

Which type of mining is generally more expensive in terms of cost-per-unit of production?
Why is surface type of mining generally more expensive?
What is generally more expensive in terms of cost-per-unit of production?
In which terms is the surface type of mining generally more expensive?
Пятый чет не могу придумать прост