With the development of the Internet, _______ communication is done by regular mail.
All the things __________ must be done well.
Follow your doctor’s advice,_____ your cough will get worse.
I spent five hours_____________ the book__________ I paid 10 dollars.
When the old man_____to walk back to his house,the sun____itself behind the mountain.
I wrote a letter of complaint(投诉),and the airline have promised to _______the matter.