Modern science has given clear evidence ________ smoking can lead to many diseases.(2011·天津，13)

__an advertisement says, remember that there is ________as a free lunch in the world.

I guess we’ve already talked about this before but I’ll ask you again just __________

______the Internet is of great help, I don’t think it’s a good idea to spend too much time on it

The toughest thing about success is ________ you have to keep on being a success