. My first summer vocation in the high school I spent a fortnight on the farm is really

问题补充：

. My first summer vocation in the high school, I spent a fortnight on the farm, is really unforgettable.

A. when

B. that

C. which

D. the one

答案：

(答案→)A. when

B. that

C. which

D. the one

解析：句意“我上高中后的第一个暑假，在农场度过的14天很难忘。”非限制性定语从句不缺少成分，先行词summer vocation指时间，故用when。故选A。