Statements: Some tapes are discs. Some discs are cassettes. Some cassettes are songs. Conclusions:  Some songs are discs. Some cassettes are tapes. Some songs are tapes. No song is a disc.

Since each combination of premises shall contain two particular premises, no definite conclusion can be drawn. However, I and IV involve the extreme terms of the second and third premises and form a complementary pair. Thus, either I or IV follows.