有个小问题啊

Private Sub Command1_Click()
Dim a As Integer
Dim b As Integera = 90 * Rnd + 10
b = 90 * Rnd + 10
Label1.Caption = a & "+" & b & "="
Command2.Enabled = TrueEnd Sub
------------------------
Private Sub Command2_Click()
Dim c As Integer
c = Text1.Text
If c = a + b Then
    MsgBox "你真聪明^_^", , "add"
    Else
       MsgBox "-_-再想想", , "add"
       End If
End Sub为什么算出来的结果都是错误的
？？？？