以下VB 程序段的功能是：单击一次按钮Command1，在列表List1 中随机输出一行10个大小写字母。已知字母“A”和“a”的ASCII码值分别为65和97。

Private Sub Command1_Click()

Dim s As String, c As String, i As String, a As String, t As Integer

Randomize

s = “”

For i = 1 To 10

              

If a <= 26 Then

t = a + 64

Else

              

End If

c = Chr(t)

s = s + c

Next i

List1.AddItem s

End Sub

以上横线处的正确语句是(  )

A、①a=Int(Rnd * 52 + 1)  ②t = a + 70

B、①a=Int(Rnd * 52)  ②t = a + 71

C、①a=Int(Rnd * 52 + 1)  ②t = a + 72

D、①a=Int(Rnd * 52)  ②t = a + 73