Basic C++ problems

Bumping cause I have another midterm.

Code:

string reverse(string s)
{
char temp;
int i = 0;
int j = s.size() – 1;
while (i < j)
{
??? WHAT GOES HERE ???
}
return s;
}

Options for ??? WHAT GOES HERE ??? are:

Code:

A.temp = s.substr(0, i)
s = s.substr(0,j) + temp;

B. temp = s.at(i);
s.at(i) = s.at(j);
s.at(j) = temp;

C.temp = s.substr(j);
s = temp + s.substr(i, j);

D.s.at(i) = s.at(j);
s.at(j) = s.at(i);

E.temp = s.at(i);
s = s.substr(i + 1);
s += temp;

6.s = s.substr(j) + s.substr(i);

The purpose of the function is to to flip around a string (i.e testing turns in gnitset).
I would assume i is incremented in this problem and that j is decremented. With that assumption, would the correct answer be B?
My logic for B is that we have a temp variable to hold of s.at(i) so that it won't be lost, and that once s.at(i) is set to s.at(j), s.at(j) is given the char stored in s.at(i) through temp. Then the loop would keep going until i = j (odd # of letters) or i = j + 1 (even # of letters).