Her grandma is here ( ) three days
Afor Bin Cat Dbefore
选项是A为什么呢..Her grandma is here ( for) three days 句意是：她的爷爷在这里【滞留/停留/呆】三天。介词in表【将来】，介词at表【时间点】，不能持续【三天】。介词before用于【过去时】