有如下VB程序段：

Dim i As Integer, sum As Integer

Dim a(1 To 6) As Integer, b(1 To 6) As Integer

sum = 0

a(6) = 18: a(5) = 7: b(6) = 14: b(5) = 16

For i = 4 To 1 Step -1

 a(i) = a(i + 2) - a(i + 1)

 b(i) = b(i + 2) - a(i)

 If b(i) Mod 3 = 0 Then sum = sum + b(i)

Next i

Text1.Text = Str(sum)

该程序段运行后，文本框Text1中显示的内容是（  ）

A、39

B、30

C、-9

D、12