问题补充：
— The light in his room is still on. Do you know ?
— To prepare for the coming exam, I think.
A. if he works hard
B. why he stays up so late
C. why is he so busy
D. when he will stop working
答案：
A. if he works hard
(答案→)B. why he stays up so late
C. why is he so busy
D. when he will stop working
解析：根据后文回答，我认为在为即将到来的考试做准备，可知上文在问你知道为什么他熬夜那么晚吗？宾语从句的结构：主语+谓语+连接词+从句。从句在句中作宾语。从句要用陈述句的语序。如果主句是一般现在时，从句根据需要选择时态，如果主句是过去时态，从句要用相应的过去时态。但是从句是客观真理，一般用一般现在时态。AD连接词不正确，C语序不正确，根据句意，故选B。
