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The term "x10^-9" in scientific notation signifies a factor associated with the metric prefix "nano-." This prefix is used to denote one billionth of a unit, which is equivalent to \(10^{-9}\). In practical terms, when referring to length, the nanometer (nm) is a commonly used unit of measurement in fields such as physics and chemistry, particularly in nanotechnology and molecular biology, where measurements on an atomic or molecular scale are often required.

For example, a nanometer is a useful unit when discussing wavelengths of light or dimensions at the molecular level, such as the size of DNA or proteins. The other choices represent different prefixes: "macro-" is associated with millions (or \(10^6\)), "micron" or micrometer is associated with millionths (or \(10^{-6}\)), and "milli-" denotes thousandths (or \(10^{-3}\)). Since "x10^-9" corresponds directly to "nano," this reinforces that the correct interpretation is indeed that it signifies nanometers.