PLZ HELP WILL MARK BRAINLIEST

Determine whether the set of whole numbers is closed under addition. explain why the set is or is not closed. give an example.

Of the ones listed here, the factor that had the greatest impact on the eventual destruction of native americans' ways of life was the development of the a) automobile. b) highway. c) railroad. d) steamboat.

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie