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

[EX Q]: Water is the only substance on earth that exists naturally in _ states .
[EX A]: three

[EX Q]: Sediment consists of _ visually distinct floc layers.
[EX A]: two

[EX Q]: Neurological surgeons undergo _ to eight years of rigorous training following medical school.
[EX A]:
six