光纤产生损耗的原因很多，下列哪一项不是其主要类型（）.

A、固有损耗

B、外部损耗

C、传输损耗

D、应用损耗

以下程序段的作用是计算数据的和，数据的个数是可变的。请填空。
Private Sub Command1_Click()
Dim a As Integer
a=0
Sum a, 1, 5, 2
Print "sum of 3 = "; a
a = 0
Call Sum(a, 1, 3, 5, 7, 9)
Print "sum of 5 = "; a
End Sub
Private Sub Sum(x As integer, 【3】
Dim y As Variant
For 【4】
x = x + y
Next y
End Sub