有如下程序段：

Dim a(1 To 6) As Integer

a(1) = 6

For i = 2 To 6

    a(i) = Int(Rnd * 30 + 1) * 3

    If a(i) Mod 2 = 1 And i Mod 2 = 0 Then

        i = i - 1

    ElseIf a(i) > 60 Then

        a(i) = a(i) - a(i - 1)

    End If

Next i

执行该段程序后，a(1)~a(6)中的值可能是 （     ）

A、6,36,15,42,72,12

B、6,14,36,51,18,21

C、6,60,3,87,0,90

D、6,18,21,75,27,21