Which sentence can i change into active voice?

The first and third sentences are pive; the second is already active. In the pive ones, think about who is doing the using and the discovering. Those people can be the subject of an active sentence. For example, in the sentence "Three death-row felons have been pardoned", you would think of who has been doing the pardoning, say, the governor. To make it active, you might say "The governor has pardoned three death-row felons."