What must be the missing logic in place of missing lines for finding sum of nodes of binary tree in alternate levels?

//e.g:-consider -complete binary tree:-height-3, [1,2,3,4,5,6,7]-answer must be 23
n=power(2,height)-1; //assume input is height and a[i] contains tree elements
for(i=1;i<=n;)
{
      //present level is initialized to 1 and sum is initialized to  0
      for(j=1;j<=pow(2,currentlevel-1);j++) 
      {
         sum=sum+a[i];
         i=i+1;
      }
   //missing logic
}

A.

i=i+pow(2,currentlevel);
currentlevel=currentlevel+2;
j=1;

B.

i=i+pow(2,currentlevel);
currentlevel=currentlevel+2;
j=0;

C.

i=i-pow(2,currentlevel);
currentlevel=currentlevel+2;
j=1;

D.

i=i+pow(2,currentlevel);
currentlevel=currentlevel+1;
j=1;

Answer: Option A