Random Math Operator

Random rnd = new Random();
int i = rnd.Next(0, 4);

int a = 0;
int b = 0;
if (int==1)
{
 int i = a + b;
}
if (int == 2)
{
 int i = a - b;
}
if (int == 3)
{
 int i = a / b;
}
if (int == 4)
{
 int i = a * b;
}

if(condition1)
{
//assign Addition;
}
else if(condition 2)
{
//assign multiplication;
}
and so on ...

private string RandomString(int size, bool lowerCase)
{
StringBuilder builder = new StringBuilder();
Random random = new Random();
char ch ;
for(int i=0; i<size; i++)
{
ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))) ;
builder.Append(ch); 
}
if(lowerCase)
return builder.ToString().ToLower();
return builder.ToString();
}

Random rndNo = new Random();
int operations = rndNo.Next(0, 4);
if (operations==1)
{
 //do stuff for addition

}
if (operations == 2)
{
 //do stuff for subtraction

}
if (operations == 3)
{
 //do stuff for multiplication

}
if (operations == 4)
{
 //do stuff for division
}