APA

Helfrich H. W., . (1970). The environmental crisis. New Haven and London: Yale University Press.

Chicago

Helfrich Harold W, . 1970. The environmental crisis. New Haven and London: Yale University Press.

Harvard

Helfrich H. W., . (1970). The environmental crisis. New Haven and London: Yale University Press.

MLA

Helfrich Harold W, . The environmental crisis. New Haven and London: Yale University Press. 1970.