British scientists have produced _____ they believe is the world’s smallest Christmas card．

British scientists have produced _____ they believe is the world’s smallest Christmas card．

A.which

B.who

C.that

D.what

答案及解析： D

解析：

考查宾语从句.从句中动词believe缺少宾语,故选D.英国科学家生产出了他们认为是最小的圣诞卡片