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

[Q]: Atoms are most stable when they have _ electrons in their outer electron energy level.
[A]: eight


[Q]: Morocco has _ perennial rivers and many dams have been built with potential for power generation.
[A]: four


[Q]: Some comets have short periods of less than _ years.
[A]:
seven