Brass gets discoloured in air because of the presence of which of the following

Brass gets discoloured in air because of the presence of which of the following gases in air?

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1) Electric current is measure by

2) Which one of the following is the petroleum wax?

3) Which of the following is an element?

4) The chemical (ethyl mercaptan) added to the otherwise odourless LPG cooking gas for imparting a detectable smell to the gas is a compound of

5) Which of the following is used for removing air bubbles from glass during its manufacture?