A rainbow is actually a complete circle, although we usually only see half of it.
Why it is true
Rainbows are complete circles formed by the refraction of light in raindrops. From the ground, we only see the upper half because the horizon blocks the lower part. From an aeroplane, it is possible to see the complete circle.
Source: NOAA
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.