A．铁在潮湿空气中生锈了

B．天然气的燃烧

C．铜在潮湿空气中产生铜绿

D．NH₄Cl与Ba(OH)₂8H₂O反应产生NH₃

下列程序的输出结果是______.
#include ＜stdio.h＞
fun ()
static int a=O;
a+=3; printf("%d",a);

main ()
int cc;
for (cc=1;cc＜5;cc++) fun();
printf ("\n");