1. (2017高一上·台州期末) 现有如下Visual Basic程序段：

Dim a(1 To 10) As Integer

a(10) = 1

a(9) = 2

For x = 8 To 1 Step -1

    a(x) = a(x + 1) + a(x + 2)

Next x

该程序段运行后，a(6)的值是（   ）