下面程序的功能是统计字符串中“i”的个数，请填空。
Sub COU( )
Dim n As Integer
a$ = "Beijing University of Technology"
n = Len(a$ )
For i = 1 To n
b$ =Mid$(a$,i, 1)
If 【11】 Then x=x + 1
Next
Print "x ="; x
End Sub