单项选择题

Compared to alternative energy, one of the disadvantages of fossil fuels is ______.

A．Mining is difficult.
B．It costs a lot of money to preserve.
C．They are difficult to be processed.
D．They are non-renewable.

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Compared to alternative energy, one of the disadvantages of fossil fuels is ______. 您的浏览器不支持 audio 元素。