句意：航班即将晚点，这并不让人感到惊讶。本题考查的是主语从句的连词的用法。本句是一个由it作形式主语的主语从句，真正的主语是横线后面的句子。句子不缺任何成份，所以应用that引导，that可以省略，故D正确。
【知识拓展】1．what引导的名词性从句，作联系代词，在从句中作主语、宾语和表语。
e．g．：(1)What she did is not yet know．她做了什么(我们)还不知道。(主语从句)
(2)1 wonder what she’s writing to me about．我想知道她给我的信中写什么了。(宾语从句)
(3)That was what she did this morning．这就是她今天早上所做的。(表语从句)
2．which引导的定语从句。
e．g．：(1)This is the book，which is written by Lu Xun．这就是鲁迅写的那本书。
(2)Tom didn’t pass the physics exam，which made his parents very angry．汤姆物理考试不及格，这使得他父母很生气。
3．how引导的宾语从句。
e．g．：(1)I don’t know how he finished his work．我不知道他是如何完成他的工作的。
(2)They want to know how to do it to help us．他们想知道怎么做才能帮到我们。
4．it引导的强调句结构。
e．g．：(1)It was yesterday that he met Li Ping．他是昨天遇到李平的。
(2)It was not until his wife came back that he went to bed．直到他妻子回家他才上床睡觉。