be going to 结构 — 英语选择题

Tom 正在写周末计划。他先写下："I am going to visit my grandparents on Saturday." 后来他改变主意，想表达“我们全家星期日将要去野餐”，下列句子正确的是（　） A. We is going to have a picnic on Sunday. B. We are going to having a picnic on Sunday. C. We are going to have a picnic on Sunday. D. We are going to have a picnic on Saturday.