【2015·陕西】Mary really hard on his book and thinks he’ll have finished it by Friday.
The doctor will not perform the operation ___ it is absolutely necessary.
Difficulties are nothing ___ we are not afraid of them.
―When do we need to pay the balance ? ― September 30.
I'd like to go swimming ____ the water is not too cold.
It’s quite me why such things have been allowed to happen.