I✀m sorry it has taken me so long to write,but............

哦,这个but是没有意思的,如果要说有,那也只能说是起强调作用.
以前做到过很多类似的题目,而且很多同学都落入了题目设下的圈套
至于补句,
I'm sorry it has taken me so long to write,but
I also need to have a good rest.

I'm sorry it has taken me so long to write,but I really tried my best to finish it as early as possible.
对长时间的歉意，以及企图让对方原谅

but 在此是转折的意思
i am sorry it has taken me so long to write,but i will try my best to finish it on time.

I\'m sorry it has taken me so long to write,but anyway I could finish it on time.
But是转折的意思

I'm sorry it has taken me so long to write, but anyway I am sitting here writing to you now.