Because if you start at one for example you starting at a extra cm/m/mm. So when you get your measurement result it will be wrong by just one cm/m/mm. Hope this wasn't too confusing.
Answer:
A. They are nonreactive gases
The density of water is a physical property.
You can measure it without changing the water to a different substance.
It is <em>not a physical change</em> because the water does not change to ice or steam.
You can observe a <em>chemical property</em> or a <em>chemical change</em> only if the water <em>changes to a different substance</em>.
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