A、According to the time the photos were taken. B、According to the subjects the photos reflect. C、According to the cultural angle

In 1958, the Swiss-born photographer Robert Frank published a book of eighty three photographs called The Americans. This book is widely considered one of the most important books of photographs published since World War II. The Americans shows the country and its people in a boldly honest way. Robert Frank showed the effects of racism, social inequality and poverty. But he also showed a beautiful side of America’s people and places.[29]The book was revolutionary for the inventive style of taking photographs that show directness and emotional honesty. The Americans was also different from other photography books because Frank organized the images according to similar emotions, forms or subjects.
    In a recent exhibition, The Americans is divided into four parts. Each part shows a different side of American culture. Each begins with a photograph that includes an American flag. In one photograph called "Charleston, South Carolina", an African-American woman holds a white baby. Frank shows the woman’s dark skin next to the baby’s light skin. You can see the wisdom and experience in her face, which is very different from the baby’s wondering expression. Here Frank plays with contrasts, or opposing elements, as he does in many photographs. The National Gallery curator Sarah Greenough organized this show. Here she talks about a different section of the exhibit: "Frank shifts his focus slightly from the American people to the physical character of the spaces they have constructed for themselves.[31]In his photographs of gas stations, diners and cemeteries, he found a beauty so simple but commonplace that it was often overlooked, so unspeakably true but deep that it was rarely acknowledged."
29. What is the most creative part of Robert Frank’s photography book The Americans?
30. How was Robert Frank’s photography book arranged in a recent exhibition?
31. What does the speaker say about Robert Frank?