Given a sentence, fill out the missing word with a 'no' or a number (between zero and ten). You should write the numbers with english alphabet, like: four instead of 4.

[Q]: Carbon atoms are tetravalent, and therefore, can have ____ bonding pairs around each carbon atom.
[A]: four


[Q]: Cleavage is perfect in ____ directions forming rhombs.
[A]: three


[Q]: Gases inhibit microorganisms by ____ mechanisms.
[A]:
two