Underground aquifers are the commonest source for well water.
Underground aquifers are the commonest source.
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The answer depends on what precisly you mean. The commonest
engineering technology involved is turbines, but there are many
ways to turn them. probably the commonest way is steam. The
commonest fuel to generate the steam, globally - coal.
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The word 'commonest' is the superlative form of the adjective 'common'. An adjective is a word used to describe a noun. Example:
The commonest kind of shoes worn by everyday people are actually designed for athletes.