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

Example Input: Optical isomers differ in the arrangement of _ groups around a chiral carbon.
Example Output: four

Example Input: Ticks are small round arachnids with _ legs and can have either a soft or hard body.
Example Output: eight

Example Input: Ligands seeks _ receptors into binding and mutual phosphorylation.
Example Output:
two