Which statement causes overflow?

int x $=3000$;

int y $=1000$;

int z;

Select one:

a$.$ z $=$ x $*$ y;

b$.$ z $=($x $*$ y$)+($x $*$ y$)+($x $*$ y$)$;

c$.$ z $=($x $*$ x$)+($y $*$ y$)$;

d$.$ z $=$ x $*$ y $*$ y;