In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

[Q]: Comets are a bit like giant dirty ice-balls with diameters between _ and fifty kilometres.
[A]: five

[Q]: Plasma levels fall by about a half within three to _ hours.
[A]: four

[Q]: Animals develop either two or _ embryonic germs layers .
[A]:
three