________ twice a year, whether it is a car or a bus or a truck, is the rule that every driver must obey in this city.
Achieving a high degree of proficiency in English as a foreign language is not a mysterious _____ without scientific basis.
They were so far away that I couldn't _____ their faces clearly.
If the boy had _____ the dog alone it wouldn't have bitten him.
The generation _____ makes it difficult for parents to understand their children's opinions.
With sufficient scientific information a manned trip to Mars should be _____.