In the early days of Indian cinema, the wealthy elite were often portrayed as the protagonists, as they were seen as the most glamorous and desirable.

Over time, the middle class became more prominent as protagonists in Indian cinema, as filmmakers began to explore their stories and experiences.

Do Bigha Zamin (1953) is often credited with popularizing the portrayal of the working class in Indian cinema, as it was one of the first films to focus on the struggles of a peasant farmer.

All of the above factors have contributed to the trend towards portraying the rural poor in Indian cinema.

The decline of the joint family is not a common theme in films about the changing class dynamics in Indian cinema.

Mother India (1957) deals with the theme of the clash between tradition and modernity, as it explores the conflict between a traditional mother and her modern son.

Shree 420 (1955) is not considered to be a classic of Indian cinema, as it is seen as a more commercial and less serious film than the other three.

Amitabh Bachchan is known for his portrayal of working-class characters in Indian cinema, as he has played a variety of roles, from a factory worker to a taxi driver.

Nargis is known for her portrayal of middle-class characters in Indian cinema, as she has played a variety of roles, from a school teacher to a housewife.

Bimal Roy is known for his films about the changing class dynamics in Indian cinema, as he has directed films such as Do Bigha Zamin (1953) and Mother India (1957).

Pyaasa (1957) is not directed by Bimal Roy, it is directed by Guru Dutt.

Pyaasa (1957) is not produced by Mehboob Khan, it is produced by Guru Dutt.

Do Bigha Zamin (1953) is not written by Guru Dutt, it is written by Bimal Roy.

Shree 420 (1955) is not scored by Ravi Shankar, it is scored by S.D. Burman.

Do Bigha Zamin (1953) is not choreographed by Saroj Khan, it is choreographed by B. Sohanlal.