The distance from the earth to the moon, FYI, is over 200,000 miles. That is a LONG distance. We are trying to find a reasonable unit of measurement.
What do we mean by "reasonable?" We mean that that unit should be able to express the distance as a number that is not huge or tiny. In other words, we want it to be as close as possible to 1.
mm, or millimeters, is a tiny unit. It is not reasonable.
cm, or centimeters, is also a tiny unit. It also is not reasonable.
m, or meters, is also a relatively small unit. It also is not reasonable.
km, or kilometers, is the largest of the four. In km, the distance from the earth to the moon is about 363,000 km. That's not too bad.
However, if these weren't the only options, I would choose Mm, or megameters, which would put the distance at 363 Mm.
