在如下的 C++ 代码中实现了对字符串中出现的 26 个字母的个数统计，横线处应填入是（ ）。

string str = "HELLO CHEN A DAI";
int strlen = str.length();
char alpha[26] = {65};
int cnt[26] = {0};
for (int i = 1; i < 26; i++)
{
    _______________;
}
for (int i = 0; i < 26; i++)
{
    cout << alpha[i] << " ";
}
cout << endl;
for (int i = 0; i < 26; i++)
{
    for (int j = 0; j < strlen; j++)
    {
        if (alpha[i] == str[j])
        {
            cnt[i]++;
        }
    }
}
for (int i = 0; i < 26; i++)
{
    cout << cnt[i] << " ";
}

{{ select(1) }}

• alpha[i] = alpha[i - 1] + 1;
• alpha[i] = alpha[i] + 1;
• alpha[i + 1] = alpha[i] + 1;
• alpha[i - 1] = alpha[i] + 1;