Knapp, B., McCrae, D., & Ross, S. (1989). Challenge of the natural environment. Longman.
Lua i Stíl Chicago (17ú heag.)Knapp, B.J, D.L.R McCrae, agus S.R.J Ross. Challenge of the Natural Environment. Essex, Eng: Longman, 1989.
Lua MLA (9ú heag.)Knapp, B.J, et al. Challenge of the Natural Environment. Longman, 1989.
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