从键盘上输入两个字符串，分别保存在变量str1、str2中。确定第二个字符串在第一个字符串中起始位置的函数是______。

A．Private Sub Command1_C1ick()
CommonDialogl. FileName="SayeFile"
CommonDialogl. Filter="All Files|*. *|(*. txt)|*. txt|(*. doe)|*. doc"
CommonDialogl. FilterIndex=2
CommonDialogl. DialogTitle="Save"
CommonDialogl. Action=2
End Sub
B．Private Sub Command1_Click()
CommonDialogl. FileName="SaveFile"
CqmmonDialogl. Filter="All Files|*. *|*. txt)|*. txt|(*. doe)|*. doff"
CommonDialogl. FilterIndex=1
CommonDialegl. DialogTitle="Save"
CommonDialogl. Action=2
End Sub
C．Private Sub Command1_Click()
CommonDialogl. FileName="Save"
CommonDialogl. Filter="All Files|*. *|(*. txt)|*. txt|(*. doe)|*. doe"
CommonDialogl. FilterIndex=2
CommonDialogl. DialogTitle="SaveFile"
CommonDia]ogl. Action=2
End Sub
D．Private Sub Command1_Click()
CommonDialogl. FileName="SaveFile"
CommenDialogl. Filter="All Files|*. *|(*. txt)|*. txt|(*. doC) |*. doe"
 

A．顶层扇出较少，中间扇出较高，底层模块低扇入
B．顶层扇出较高，中间扇出较少，底层模块高扇入
C．顶层扇入较少，中间扇出较高，底层模块高扇入
D．顶层扇入较少，中间扇入较高，底层模块低扇入