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

Example Input: Diatoms look like _ shells that fit together.
Example Output: two

Example Input: Ticks mature in _ stages, moving from egg to larva and nymph to adult.
Example Output: four

Example Input: Salmonids use _ forms of cover, overhead and submerged.
Example Output:
two