Help with unassigned local variable

<pre>

float exams = float.Parse(txtboxexams.Text.Trim());
            float termpaper = float.Parse(txtboxtermpaper.Text.Trim());
            

            float examgrade;
            float termpapergrade; 



            examgrade = exams / 250;
            termpapergrade = termpaper / 50;

            string lettergrade;


            if (radbutunder.Checked == true)
            {
                if (examgrade >= 0.93)
                {
                    if (termpapergrade >= 0.93)
                    {
                        lettergrade = "A";
                    }
                    else lettergrade = "A-";
                }
                else if (examgrade >= 0.9)
                {
                    if (termpapergrade >= 0.9)
                    {
                        lettergrade = "A-";
                    }
                    else lettergrade = "B+";
                }
                else if (examgrade >= 0.87)
                {
                    if (termpapergrade >= 0.87)
                    {
                        lettergrade = "B+";
                    }
                    else lettergrade = "B";
                }
                else if (examgrade >= 0.83)
                {
                    if (termpapergrade >= 0.83)
                    {
                        lettergrade = "B";
                    }
                    else lettergrade = "B-";
                }
                else if (examgrade >= 0.8)
                {
                    if (termpapergrade >= 0.8)
                    {
                        lettergrade = "B-";
                    }
                    else lettergrade = "C+";
                }
                else if (examgrade >= 0.77)
                {
                    if (termpapergrade >= 0.77)
                    {
                        lettergrade = "C+";
                    }
                    else lettergrade = "C";
                }
                else if (examgrade >= 0.73)
                {
                    if (termpapergrade >= 0.73)
                    {
                        lettergrade = "C";
                    }
                    else lettergrade = "C-";
                }
                else if (examgrade >= 0.7)
                {
                    if (termpapergrade >= 0.7)
                    {
                        lettergrade = "C-";
                    }
                    else lettergrade = "D+";
                }
                else if (examgrade >= 0.67)
                {
                    if (termpapergrade >= 0.67)
                    {
                        lettergrade = "D+";
                    }
                    else lettergrade = "D";
                }
                else if (examgrade >= 0.63)
                {
                    if (termpapergrade >= 0.63)
                    {
                        lettergrade = "D";
                    }
                    else lettergrade = "D-";
                }
                else if (examgrade >= 0.6)
                {
                    if (termpapergrade >= 0.6)
                    {
                        lettergrade = "D-";
                    }
                    else lettergrade = "F";
                }

                lblfinalgrade.Text = lettergrade;