a,b,c are global variable a=0,b=0,c=0
main() assigns value to a=3,b=5,c=7.
it calls func1()
func1()
it create local variable a=6,b=8,
Calls func2()
func2()
it create local variable b=4, c = a+b
as func2() has no local variable named a,
global a is available to func2().
c=3+4 = 7
global a = 3+11 =14
print(c) ----------- 7
return to func1()
func1():
a=a+b+c
a,b are local and c is global
a=6+8+7 = 21
print(a) local a ----------- 21
retun to main()
