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Metre.
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You would use millimeters (mm) to measure the thickness of a house key, as it is a small unit appropriate for measuring such dimensions.
meters
metres
kilo-newtons.
I would use inches or centimeters, or millimeters to measure this.
You would use the metric unit of millimeters (mm) to measure the size of a common house fly.
usually heated square feet. That means actual living area.
Lengths are measured in meters.
I would go with millimetres
Meters or feet are commonly used to measure the width of a house.