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In C, every program execution starts from main and ends with main. So the first function executed in main it self. So no need to prototye main function, if we change the return type.

Actually in C, every function retuns integer defaulty.
But not every function must return integer.
we can change the return type.
Main is also an function, so we can change the return type.

main()
{
int i=1,m,j=1,n,k;
clrscr();
scanf("%d",&n);
for(m=1;m<=n-2;m++)
{
i=j;
j=k;
k=i+j;

## is String concatenation operator. This will concatenates two values in that Macro.
Here Var12=100.

In Macro f(g1,g2) is g1##g2.

So If f(var,12) then it is var12 so Answer is 100.
If we put...