单项选择题

下列程序
Private Sub form_Click()
score=Int(Rnd * 10)+80
Select Case score
Case Is＜60:a$＝"F"
Case 60 To 69:a$＝"D"
Case 70 To 79:a$＝"C"
Case 80 To 89:a$="B"
Case Else:a$＝"A"
End Select
Print a$
End Sub
运行时输出的结果是