VB下标越界

各位大佬，我运行为什么下面程序会出现下标越界，跪求答案
Private Sub Command1_Click()
Dim a!, b!, c!
Dim i(1 To 6)  As Single
Dim l(1 To 6)  As Single
Dim h(1 To 6)  As Single
Dim pd(1 To 6) As Single
Dim pc(1 To 6) As Single
Dim bg(1 To 6) As Single
For a = 1 To 6
   i(a) = Val(InputBox("输入第" & a & "段线路坡度"))
   l(a) = Val(InputBox("输入第" & a & "段线路长度"))
   h(a) = Val(InputBox("输入第" & a & "段线路标高"))
Next a
c = 1
b = 1

Do While c < 7

      pd(b) = (h(c + 1) - h(c)) * 1000 / (l(c) + l(c + 1))
   
   If l(c) * Abs(pd(b) - i(c)) <= 2000 And l(c + 1) * Abs(pd(b) - i(c + 1)) <= 2000 Then
 
        pd(b + 1) = (h(c + 2) - h(c)) * 1000 / (l(c) + l(c + 1) + l(c + 2))
   
      If Abs(pd(b + 1) - i(c)) * l(c) <= 2000 And Abs(pd(b + 1) - i(c + 2)) * l(c + 2) <= 2000 And Abs(pd(b + 1) - i(c + 1)) * l(c + 1) <= 2000 Then
   
         
         Label1.Caption = Label1.Caption & pd(b + 1) & vbCrLf
      
         Label2.Caption = Label2.Caption & (l(c) + l(c + 1) + l(c + 2)) & vbCrLf
         
         c = c + 3
         
         b = b + 1
      Else
         Label1.Caption = Label1.Caption & pd(b) & vbCrLf
      
        Label2.Caption = Label2.Caption & (l(c) + l(c + 1)) & vbCrLf
         
         c = c + 2
         
         b = b + 1
       End If
   Else
   
       Label1.Caption = Label1.Caption & i(c) & vbCrLf
      
        Label2.Caption = Label2.Caption & l(c) & vbCrLf
        
         c = c + 1
         
         b = b + 1
   End If
  
Loop

End Sub