I would like to join a music club because I want to practice _____ songs.[ ]

A. sings

B. sing

C. singing

D. sang

下列程序计算Sn的值。Sn=a+aa+aaa+…+aaa…a，其中最后一项为n个。
例如：a=5，n=4时，则Sn=5+55+555+5555。请在空白处填入适当的内容，将程序补充完整。
Private Sub Comnmnd1_Click()
Dim a As Integer，n As Integer，Cout As Integer
Dim Sn As Long，Tn As Long
Cout=1
Sn=0
Tn=0
a=InputBox(“请输入a的值：”)
n=InputBox(“请输入n的值!”)
Do
Tn=Tn * 10 + a
Sn=Sn+Tn
Cout=Cout+1
【9】
Debug. Print a，n，Sn
End Sub