“ Although ,” the boy said , “ _______ over and over , the maths problem remained confusing to me ” .

A．explained

B．explain

C．explaining

D．having explained

稽核系统中发票销项数据由防伪税控开票子系统自动生成，并由企业向税务机关进行电子申报，发票进项数据由（）自动生成。