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

[EX Q]: Conserved residues make similar contacts in _ repressor-operator complexes.
[EX A]: two

[EX Q]: Pelycosaurs are a paraphyletic group of _ primitive families of synapsids.
[EX A]: six

[EX Q]: Krill reach maturity after _ to three years.
[EX A]:
two