没有使用foreach循环就可以针对以下问题定义LINQ语句吗？

public class GroupObject
{
    public String name;
    public List<String> letters;

    public void test()
    {
        List<GroupObject> myGroups = new List<GroupObject> {
            new GroupObject {
                name="test1",
                letters=new List<String>{"x","y","z"}
            },
            new GroupObject {
                name="test2",
                letters=new List<String>{"l","m","n"}
            },
            new GroupObject {
                name="test3",
                letters=new List<String>{"m","x","z"}
            }
        };

        // LINQ problem 1: how to get a list of all 9 letters
        // List<string> allLetters = (from ...).ToList();

        // LINQ problem 2: how to get a list of all 6 DISTINCT letters
        // List<string> allDistictLetters = (from ...).ToList();
    }
}

在写这个问题时，我找到了解决问题的办法。我仍然会发布（并回答）这个问题，因为这个问题对我来说真是个麻烦。而且我在这里没有找到合适的现有问题。

乔，尤文