看一个例子：from p in Products group p by p.Family into g select g
那么：
1: from p in Products group p by p.Family into g select g.First()
2: (from p in Products group p by p.Family into g select g).First()
什么结果呢？
from p in Products group p by p.Family into g select g.First()：
(from p in Products group p by p.Family into g select g).First()：
