Which of the following is precipitation reaction (solid formation)?
a) 2HNO₃ + CaOH₂ --> 2H₂O + CaNO₃₂
b) KCl + NaNO₃ --> KNO₃ + NaCl
c) 4Na + O₂ --> 2Na₂O
d) KBr + AgNO₃ -> KNO₃ + AgBr