Answer
A and C
Work Step by Step
A) Changes in the oxidation number:
$Zn/Zn^{2+}$: $Zn :0\rightarrow+2$
$NO_3^-/NO_2$: $N :+5\rightarrow+4$
B) No changes in oxidation numbers.
C) Changes in the oxidation number:
$Ca/Ca(OH)_2$: $Ca: 0\rightarrow+2$
$H_2O/H_2$: $H: +1\rightarrow0$
