Experiments of this kind _______in both the U.S. and Europe well before the Second World War started.
It's not enough. We need ________ more than that.
The terrible weather ________ our difficulty in building the road.
—Yesterday we learnt the fifth, ________? —I'm sorry, Mr. Li. I don't understand everything.
Wouldn't it be ________ wonderful world if all nations live in________ peace with one another?
—How was your recent trip to Sichuan? —I've never had ________ one before.