APA
Christophersen P., . (1973). Second-language learning. Harmondsworth: Penguin Education.
Chicago
Christophersen Paul, . 1973. Second-language learning. Harmondsworth: Penguin Education.
Harvard
Christophersen P., . (1973). Second-language learning. Harmondsworth: Penguin Education.
MLA
Christophersen Paul, . Second-language learning. Harmondsworth: Penguin Education. 1973.