----Do you doubt _______ she can run the race?---- Yes, I doubt _______ she can run against Liu Xiang
It's not easy to change habits,________ with awareness and self-control,it is possible.
There's no way of knowing why one man makes an important discovery________another man,also intelligent,fails.
It was a real race________time to get the project done.Luckily,we made it.
A serious study of physics is impossible________some knowledge of mathematics.
I have an appointment ________ Dr.Smith, but I need to change it.