很自然，人们会优先开发最便宜的能源，例如沙特阿拉伯易于开采的石油储备。
It was only natural for mankind to exploit the cheapest energy sources first, such as easy-to-extract oil reserves under Saudi Arabia.
如果宇航员有一天返回月球，在这种不是那样寒冷、黑暗的环境，将更易于开采水。
If astronauts ever do manage to get back to the moon, they should find it easier to mine for water in these somewhat less frigid and dark conditions.
塞拉俱乐部曾预言：“易于开采的石油越来越少，每桶60美元的原油将不复存在了。
The Sierra Club predicted: 'We're running out of oil we can pump from the ground and market for under $60 a barrel.
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