--I am going to the school party.--________ A.So am I B.so will I 答案说是B,不可能吧,应该是A吧

①I am staying in tonight,so am I.
②I am going to the school party,yeah so am I.
所谓答案有待商榷,例句①是美剧中原文.
简析：
a、so+系动词+主语.例 I was shocked,so was I.
（注意①②中be doing表将来,讲述接下来要发生的事情.不存在导入will作为代动词的说法.）
b、so+do（代替谓语动词）+主语.例 I found it very hard.so did I.
c、so+情态动词（如could）+主语,例 I'd like a cup of coffee.so would I.