阅读理解，根据短文选择正确的一项。

    One day Mr. and Mrs. White went shopping by car. They stopped their car near a store. They bought a lot of things and they wanted to put the things in the car. But Mr. White couldn't open the door of the car, so they asked a policeman to help them. The policeman was very friendly to help them. Just then a man came up and shouted:“ What are you doing with my car?” Mr. and Mrs. White took a look at the car's number and they were frozen（僵住了） there. It wasn't their car.

《国语》讲“祀，国之大节”。有学者认为，青铜器在商周时期被视为“政治的权力”。可推断，商周时期青铜器主要用作（    ）