Which one of the following is a double-displacement reaction?
A. Zn (s) + 2HCl (aq) - H2 (g) + ZnCl2 (s)

B. Ba(OH)2 (aq) + 2HClO4 (aq) - Ba(ClO4)2 (aq) + 2H20 (1)

C. 2503 (g) - 2502 (g) + 02 (g)

D. K2CO3 (s) - K20 (s) + CO2 (g)