Today is the sixth of May. It is Nancy's birthday. She is eleven years old. There is a birthday party in the evening in her home. Many friends come. They all have a lot of fun at the party. Helen is listening to music. Yang Ling is playing the piano. Wang Bing and Mike are playing computer games. Now, it is time for the birthday cake. All the children sing “Happy Birthday” together for Nancy. Then, Nancy makes a wish, blows out the candles and cuts the cake. Everyone enjoys the party very much.

地壳里含量最多的金属元素是（  ）