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All answers having "Energy source" as clue:

3 Character Answers:

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GAS

OIL

RHO

SOL

SUN



4 Character Answers:

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ATOM

CARB

COAL

FOOD

FUEL

RAYS



5 Character Answers:

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ATOMS

CARBS

STEAM



7 Character Answers:

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GASPIPE

GLUCOSE



8 Character Answers:

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COALMINE

SNACKBAR



9 Character Answers:

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COALFIELD



10 Character Answers:

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GRANOLABAR

POWERPLANT



12 Character Answers:

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NUCLEARPOWER