单项选择题
You are creating a Windows Forms application for a courier company by using the .NET Framework 3.5.You create a form that allows customers to track the progress of their shipments.The form contains the following elements：
（1）A text box named txtTN that allows users to enter a tracking number
（2）An ErrorProvider control named ErrorProvider1 that informs users of an invalid tracking number
（3）A function named ValidTrackingNumber that validates tracking numbers
You need to ensure that the txtTN text box is validated， which code segment should you use？（）
A.A
B.B
C.C
D.D
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