I found the homework was much more _____ i used to have.

I found the homework was much more _____ i used to have.
应该填than what,但不明白为什么不填than that?
He is not interested in those texts,_____are too difficult to him.
应该填most of which,为什么不填most of them或most of that?
The bird was lucky to miss____.
为什么应该填being shot?
还有两题
Beijing,_____the 2008 Olympic Games,calls on all its citizens to learn English.
应该填perparing for hosting,为什么不填are perparing for hosting?
with online course Lynn has over 300 students all over China ______her class through the Internet.
空格填attending，