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

[EX Q]: Plankton has _ known ancestors.
[EX A]: two

[EX Q]: Hoatzin has typical foot with _ toes forwards and one backwards.
[EX A]: three

[EX Q]: Esters have _ carbon chains separated by an 'ether' oxygen.
[EX A]:
two