A proposal has been put forward at a recent video conference _______ measures be taken to raise the citizens’ awareness of traffic safety.

It is well known to us all ____ China sent up a manned spaceship to outer space successfully last year.

My pen friend and I keep in touch ________ sending e-mails often

下列各句中，没有语病的一句是 （ ）

So difficult ___it to work out the problem that I decided to ask Tom for advice.