What does the speaker say about the artists in the United States during the Great Depression

A．Many artists lost faith in the value of art.

B．Many artists were forced to take jobs in other fields.

C．Many artists moved away from large cities.

D．Many artists in the United States moved to other countries.

从表中选择字段形成新关系的操作是

A．选择

B．连接

C．投影

D．并