Determining distance and location

The cubit was defined as the distance from the elbow to the tip of the middle finger. This length differs from person to person — a tall person has a longer cubit than a shorter person.

If two people use their own body to measure the same object, they will get different numbers. This makes it impossible to reliably compare or reproduce measurements across different people or places.

The core reason for moving to standardized units was that body-based measurements varied between individuals, making them unreliable for consistent scientific or commercial use.

The question asks for the main reason why civilizations moved away from body-based measurement systems toward standardized units.

Body-based measurements like the cubit are actually very easy to remember — you just look at your own arm. This option is incorrect.

There is no evidence that body-based measurements were restricted to building. The cubit was used widely across many contexts. This option is a distractor.

This is vague and inaccurate. Standardized units were developed out of practical need for consistency, not merely a scientific preference for mathematics.

The remaining option — that body-based measurements varied from person to person — is the correct reason. A cubit measured by a pharaoh differed from one measured by a shorter worker, making standardization necessary.

The prefix kilo always means $\times 1000$. Therefore, $1\text{ km}=1000\text{ m}$.

$3.5\text{ km}=3.5\times 1000\text{ m}=3500\text{ m}$

The correct conversion is 3500 m.

We need to convert kilometres to metres. The key fact is $1\text{ km}=1000\text{ m}$.

350 m would result from multiplying by 100 instead of 1000. This confuses 'kilo' ($\times 1000$) with 'hecto' ($\times 100$).

35 m would result from multiplying by only 10. This is far too small for a kilometre conversion.

0.0035 m would be the result of dividing by 1000 rather than multiplying — this is going in the wrong direction (from metres to kilometres, not the other way around).

$3.5\times 1000=3500\text{ m}$ is the only correct answer.