Hard metals can be easily cut with grinding wheels.
硬金属可以很容易用砂轮磨削。
If there were no heat-treatment, metals could not be made so hard.
没有热处理，金属就不会变得如此硬。
It is hard to think of a world without metals.
很难想象一个世界没有金属。
It's hard to think of a world without metals.
没有金属的世界是难以想象的。
It's hard to think of a world without metals.
没有金属的世界是难以想象的。
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