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

[EX Q]: Bifocal lenses have _ distinct segments with different powers.
[EX A]: two

[EX Q]: All butterflies have _ pairs of legs.
[EX A]: three

[EX Q]: Malaria is caused by _ species of Plasmodium, a unicellular protozoan.
[EX A]:
four