Tate, Merle W.
McMillan , 1955
Buku Teks
London: Her Majesty's Stationary office, 1987,
Buku Referensi
[publisher not identified], 1977
Buku Teks
Billington, E.W.
McGraw-Hill International Book Company, 1981
Buku Teks
Charvat, Frank J.
Heath, 1966
Buku Teks