The second paragraph mentions all of the following as necessary elements in the creation of steam caves EXCEPT ______.

A．The south lost the war partly because it lacked popular support.
B．The economy of the south was flexible enough to support the war efforts.
C．The North owed its victory partly to the adjustability of its economy.
D．The economic structure in the North hindered the logistics of the army.

A．home front
B．battlefield
C．government
D．economy