Which of the following are the example of double salt?
A. \( \mathrm{FeSO_4\cdot (NH_4)_2 SO_4\cdot 6H_2O} \)
B. \( \mathrm{CuSO_4\cdot 4NH_3\cdot H_2O} \)
C. \( \mathrm{K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O} \)
D. \( \mathrm{Fe(CN)_2\cdot 4KCN} \)
Choose the correct answer: