What determines the strength of a secret key within a symmetric key cryptosystem?

A. A combination of key length, degree of permutation, and the complexity of the data- encryption algorithm that uses the key
B. A combination of key length, initial input vectors, and the complexity of the data- encryption algorithm that uses the key
C. A combination of key length and the complexity of the data-encryption algorithm that uses the key
D. Initial input vectors and the complexity of the data-encryption algorithm that uses the key