21. The main idea of the passage is that:

Answer and Explanation

Your Answer is

Correct Answer is B

Explanation

The first four paragraphs talk about the advantages of photography, the fifth and sixth paragraphs mention the controversies of photography, and the last paragraph affirms the advantages of photography.

Item B: Photography is a very powerful but also controversial image form, which is in line with the theme of the whole article.

Item A: only mentions negative, wrong.

Item C: It’s just a detail, not the gist.

Item D: There is no mention of replacing other art forms.