题干
By the time I reached home , my parents ____ the dinner for me already
A：had cooked
B：cooked
C：have cooked
D：cook
答案(点此获取答案解析）
A
By the time I reached home , my parents ____ the dinner for me already
A：had cooked
B：cooked
C：have cooked
D：cook
的前
项和为
，若
，
，则
等于（ ）