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

[Q]: Diatoms are in _ shells and make the water brown.
[A]: two


[Q]: Plants grow to _ feet tall with feathery, smoky purple foliage.
[A]: four


[Q]: Air layering is a technique invented by the Chinese _ thousand years ago.
[A]:
four