Knapp, B., McCrae, D., & Ross, S. (1989). Challenge of the natural environment. Longman.
Dyfyniad Arddull ChicagoKnapp, B.J, D.L.R McCrae, and S.R.J Ross. Challenge of the Natural Environment. Essex, Eng: Longman, 1989.
Dyfyniad MLAKnapp, B.J, et al. Challenge of the Natural Environment. Longman, 1989.
Rhybudd: Mae'n bosib nad yw'r dyfyniadau hyn bob amser yn 100% cywir.