### Video Transcript

Each of the following is a unit of
distance. Which one is the largest? Is it (A) kilometer, (B) meter, (C)
light-year, (D) mile, or (E) millimeter?

Let’s start with options (A), (B),
and (E), each of which includes meter. Now, option (B) just says meter,
but (A) and (E) include the prefixes kilo- and milli-. It’ll be helpful to recall that the
prefix milli- means one thousandth, so a millimeter is one thousandth of a
meter. Another way to say this is that
there are 1000 millimeters in a meter. Since the millimeter isn’t as large
as the meter, we should eliminate option (E).

Moving on, the prefix kilo- means
1000, so one kilometer equals 1000 meters. Because the kilometer is larger
than the meter, we should eliminate option (B). The kilometer is a unit often used
in everyday life. For example, one kilometer is
roughly the length of a large suspension bridge that we might drive a car over.

Moving on, option (D) gives us
another unit that we might recognize from everyday life, the mile. Miles and kilometers are often used
in the same context because they’re of roughly the same magnitude. A mile is equal to about 1.6
kilometers, so the mile is a little bit larger than a kilometer. Thus, we should eliminate option
(A).

Now, option (C), the light-year, is
a unit that we don’t really use in daily life. Rather, we use light-years when
we’re talking about space. That’s because this unit of
distance is incredibly large. Nothing that we interact with here
on Earth even comes close to being as large as a light-year. A light-year is defined as the
distance that a beam of light would travel in one year if nothing were in its
way.

It can be difficult to understand
how fast the speed of light truly is, so consider the following. When astronauts travel to the moon,
it takes about three days for their spacecraft to complete the journey from
Earth. In comparison, it takes light less
than two seconds to travel the same distance. So just how far can light travel
over the span of a whole year? That’s one light-year, or about six
trillion miles. Therefore, we know that the
light-year is the largest unit of distance listed here, so option (C) is
correct.