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

[EX Q]: Geckos have _ eyelids .
[EX A]: no

[EX Q]: Tin exists in _ allotropic forms.
[EX A]: three

[EX Q]: Tibetan medicine defines _ main systems which control the body's processes.
[EX A]:
three