10个空填什么?最好能附上解说.

10个空填什么?最好能附上解说.
Do you have experience with online classes?I must admit at first I found online classes quite uncomfortable!Although I used 1.computer every day,I didn't get used to chatting for 2.long time.So when I took part in 3.group's scheduled weekly chats,I had 4.hard time keeping up.By 5.time I'd finished typing in my responses,I'd missed about 6.dozen comments.And by 7.time I'd answered 8.question,we were already on to 9.third topic.To solve the problem,I decided to write down the main points of the topics for 10.coming weekly chat and preview 11.lessons.This is where I began to see 12.real help of the online environment.I was able to log in any time of day or night to review 13.homework,to see other students' comments,or to add my own comments to 14.week's lecture.Most of the students in our study group were from other cities or even other countries.Their different views certainly made 15.class interesting!Overall,I found the class enjoyable and I felt I learned a lot.
都是填冠词的。
这个练习的答案给的是第9个是a，第10个是the。但我总觉得不确定。

貌似都是冠词,And by the time I'd answered a question,we were already on to the third topic.To solve the problem,I decided to write down the main points of the topics for the coming weekly chat and preview the lessons.
没有什么理由,序数词完整的结构前面就是需要加定冠词,for the coming weekly chat,这个也是缺冠词.
补充：貌似我确实错了,没能体会这个意思.因为前面说“别人在讨论某个问题的时候,我尝试去输入我的回答；此时别人在讨论,而我错过了别人的讨论时间；当我回答完问题的时候,我们已经到了又一个话题”.仔细体会这里并没有提及确定的某个第二个话题,而只是1、先回答问题,2、讨论问题,3、又一个话题.我想这里的 a third就是不特指的意思,所以需要把两个意思都体会一下,选择最适合的.语言就是选择的艺术,这个确实有点难.