The accident is ________ reminder of the potential dangers involved in ________ oil production in this area.
The fisherman made it a _____ to cast his net four times a day.
The number of people invited _____ fifty, but a number of them _____ absent for different reasons.
It was raining hard at that time. _____, we had to leave as soon as possible.
What he suggested _____ a good suggestion. Let’s put it into practice.
The Great Wall winds its way _____ mountains _____ valleys and _____ deserts to the east.