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

[EX Q]: Copper has _ stable isotopes.
[EX A]: two

[EX Q]: Binary distillation separates _ chemicals.
[EX A]: two

[EX Q]: Biology is a survey of the diversity of life, including all _ King doms, viruses and prions.
[EX A]:
six