Underground mines produce smaller amounts of coal per year than strip mines by their very nature. ...

Students could attempt to answer this question in many different ways. Some
examples of answers I consider valid are given below.
(1) The question is too vague to answer. What could by their very nature mean? (There
is justification for that, as it was my intention in using this phrase to get students to think
about exactly this issue, among others.)
(2) The question has not specified the resources available in the mines in question. One
might have much more coal available than another. Therefore, it cannot be answered.
(3)Assume we compare mines having equal amounts of coal (as estimated). We know much
more coal will be left in the ground in an underground coal mine than a strip mine. One
could argue that this means that production will be lower.
(4) Given the assumptions in (3) above, the student could argue that we have not specified
what kinds of equipment we are planning to utilize. For example, there might be just one
power shovel in use in the strip mine and there might be several longwallers in the
underground mine. The difference in mechanization would surely lead to the underground
minein this caseoutproducing the strip mine. The opposite case could occur. An
underground mine mined by individual miners with a pick and shovel would surely produce
less salable coal than a mechanized strip mine.