答案：B

题目分析：考查从句，从句competitors, usually children, are asked to spell as many words as possible.不缺少成份，所以在关系副词里选择引导词，所以选B

点评：从句是高中阶段重点，又是难点，是高考常考内容之一，本题把主语和其他的成份用逗号隔开来迷惑考生，切记不要掉入命题人的陷阱，要求考生多注意

即学即练：That art center is _______ used to be a factory, ______ millions of tractors were produced.

A. where; where    B. what; where     C. what; which         D. where; which