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

[Q]: Dairy cows live for _ generations, calve and produce normal milk on a protein-free diet.
[A]: six


[Q]: Human sperm comes in _ varieties, mixed.
[A]: three


[Q]: Most salamanders have only _ front toes but five rear toes.
[A]:
four