下列程序的执行结果为( )。 Private Sub Command1_Click() 　　Dim s1 As String,s2 As String 　　s1="abcdef" 　　Call Invert(s1,s2) 　　Print s2 End Sub Private Sub Invert(ByVal xstr As String,ystr As String) 　　Dim tempstr As String 　　i=Len(xstr) 　　Do While i＞=1 　　　tempstr=tempstr+Mid(xstr,i,1) 　　　i=i-1 　　Loop 　　ystr=tempstr End Sub

A．Private Sub Text1_KeyPress(KeyAscii As Integer) 　　If KeyAscii＜65 Or KeyAscii＞90 Then 　　　MsgBox"请输入大写字母" 　　　KeyAscii=0 　　End If 　End Sub
B．Private Sub Text1_KeyDown(KeyCode As Integer,Shift As Integer) If KeyCode＜65 Or KeyCode＞90 Then 　　MsgBox"请输入大写字母“ 　　KeyCode=0 　　　End If End Sub
C．Private Sub Text1_MouseDown(Button As Integer,Shift As Integer,X As Single,Y As Single) If Asc(Text1.Text)＜65 Or Asc(Text1.Text)＞90 Then 　　MsgBox"请输入大写字母" 　End If End Sub
D．Private Sub Text1_Change() 　　If Asc(Text1.Text)＞64 And Asc(Text1.Text)＜91 Then 　　　MsgBox"请输入大写字母" 　　End If End Sub

A．Left(c,n)
B．Right(c,n)
C．Mid(c,n,1)
D．Mid(c,n,n)