Is it possible for a change of a single nucleotide in DNA to have an observable effect on phenotype?

How did industrial production improvements impact export-dependent areas of the world during the great depression?   a.  demand for certain raw materials rose.      b.  demand for certain raw materials fell.      c.  the new technology was exported to most raw material producing nations.      d.  there was a decrease in tariffs.

Which events are correlated but do not necessarily have a causal relationship?

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