vb中的select case语句的问题

Select Case a
Case Is <睁誉 0
MsgBox "侍早悉老乎错误1！"
Case Is < 6
MsgBox "a"
Case Is < 11
MsgBox "b"
Case Is < 16
MsgBox "c"
Case Is > 15
MsgBox "d"
Case Else
MsgBox "错误2！"
End Select

select case a
case 1 to 5
A
case 6 to 10
B
case 11 to 15
C
case is>渣昌谨如基迅碰15
D
end select

if (a>0 and a<6) then num=1
if (a>5 and a<11) then num=2
if (a>10 and a<16) then num=3
if (a>15) then num=4

select case num
case 1:执行A
case 2:执行B
case 3:执行C
case 4:执世肆滚枣行搜备轿D
end select