一定概率选中某一字母

char chRet = '\0';
Random rand = new Random();
int randNum = rand.Next(100);

if (randNum < 10)
{
chRet = 'A';
}
else if (randNum < 10 + 10)
{
chRet = 'B';
}
else if (randNum < 10 + 10 + 35)
{
chRet = 'C';
}
else if (randNum < 10 + 10 + 35 + 25)
{
chRet = 'D';
}
else
{
chRet = 'E';
}