can someone please help me with my hw?

im currently taking a beginners C# course and need some pointers on the hw. I have a very hard nosed instructor that refuses to give much help..

the objective is to get a "mountain of alphabet" given the inputs and he wants us to get the following output:

a
b c
d e f
g h i j
k l m n o
p q r s
t u v
w x
y

Inputs:

Enter a letter between 'a' and 'z':

Enter a positive integer:


Here's what i have so far:
{
char sc = Convert.ToChar(TbxLetter.Text.Trim()[0]);

int n = Convert.ToInt32(TbxInteger.Text);


if (sc < 'a' || sc > 'z')
{
TbxOutput.Text = "Please enter a letter from 'a' to 'z'!!";
return;
}

TbxOutput.Text = "";



int row = 1, col = 1;

for (int i = 1; i <= n; i++)
{
TbxOutput.Text += sc.ToString() + " ";
sc++;
if (sc > 'z') sc = 'a';

if (row == 10) row = 1;

if (col == row)
{
TbxOutput.Text += "\n";
row++;
col = 0;
}

col++;

Here's the Ouput that i get:

a
b c
d e f
g h i j
k l m n o
p q r s
t u v w x
y z a b


what do i need to do to make it continually decrease until it gets to Y and then make it start over again at Z?

any pointers would be helpful

Thanks in advance.