问题补充：
－When should I hand in my paper?－It after class.
A. must be handed in
B. is handed in
C. was handed in
D. will hand in
答案：
(答案→)A. must be handed in
B. is handed in
C. was handed in
D. will hand in
解析：句意：我应该什么时候交卷？——下课后必须交。结合语境可知下文描述的是客观性动作，选项B只是陈述事实，语气不如选项A强烈，故选A.
