Why was Mr Lin late for the meeting this morning?
Because he didn't know (B.when the meeting would start).
为什么马先生今早开会迟到了?因为他不知会议几点开始.
括号中要填的是个宾语从句,要用陈述句的语序,即主语在动词之前.其他两项都是疑问语序,不能选.