You have added an irreversible inhibitor to a sample of enzyme and substrate. at this point, the reaction has stopped completely. what is the best way to get the activity of the enzyme back up?

To add new enzyme molecules to the system.

Explanation:

An irreversible inhibitor is the one that either binds to the enzyme by covalent linkage or damages its functional groups which are otherwise vital for the activity of the enzyme. Alternatively, these inhibitors can form multiple non-covalent interactions with enzyme and form the transition site analogs.

The tight binding of these inhibitors to the enzymes does not allow the substrate molecules to form the enzyme-substrate complex and the reaction is stopped completely.  

The tight binding of irreversible inhibitors to the enzymes can not be reversed and mostly renders the enzymes non-functional. To allow the reaction to occur, it is required to add the new enzyme molecules to the reaction mixture.

b. the carbon cycle ensures living things have a source of hydrogen

oxygen +1 most likely