Definition: In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.
Input: Michael Westen:  Creating an explosive device in the field requires a strong knowledge of chemistry and the ability to improvise with whatever equipment happens to be lying around ... An old fire extinguisher makes an ideal compression chamber . Add some metal shavings and the right cleaning supplies , and you can serve up a bomb that can blast through a cinderblock wall .
Fiona Glenanne: Why is it you can never find a good sodium hydroxide solution when you need _ one _ ?

solution