What is 0.000345 expressed in scientific notation?

To write 0.000345 in scientific notation, move the decimal point 4 places to the right to get 3.45. Because the original number is less than 1, the exponent is negative, so 0.000345 = 3.45 × 10⁻⁴. The correct choice is 3.45 × 10⁻⁴.

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