填空题

下面这个Sub过程的功能是统计字符串中“a”的个数，请在空白处填上合适的代码完成程序。
Private Sub numCount()
Dim hum As Integer
s＄ = "software And hardware"
num = Len(s$)
For i = 1 To hum
b$=______
If b$ = "a" Then x = x+1
Next i
Print "x="; x
End Sub