My visit to your school is really an unforgettable experience I will treasure for ever.A

问题补充：

My visit to your school is really an unforgettable experience, I will treasure for ever.

A. that

B. one

C. it

D. what

答案：

A. that

(答案→)B. one

C. it

D. what

解析：B句意：我到你们学校的参观确实是一次令人难忘的经历。我将永远珍惜这样的经历。根据句意，此处应用one作an unforgettable experience的同位语，后面是省略that或者which的定语从句。