Which one of the following fishes is a surface feeder?

What are Rabi and Kharif crops? Give two examples each.

Kavita,the science teacher,was teaching her students about the types of diseases based on duration: acute and chronic diseases. She also discussed their ability to spread in public and their immediate causes.
$(a)$ What are acute and chronic diseases? Give one example of each.
$(b)$ Name the type of diseases which spread by coming in contact with a sick person. Name the different types of organisms which cause diseases.

Ram tried hard to push the wall of a room. In the process of pushing the wall,he begins to perspire and feels exhausted although the wall does not move. Ram feels that he has done a lot of work and spent a lot of energy. Do you think that Ram was justified in making the statement that he has done a lot of work? Justify your answer and give the conditions under which work is done.

Vividha told Vasudha that she will show her some magic. Vividha took a metallic pipe. She asked Vasudha to hold one of its ends close to her ears. She gently tapped the other end of the pipe with a metallic object. She asked Vasudha what she heard. Vasudha said she heard two sounds.
$(a)$ Was this magic? Why did Vasudha hear two sounds?
$(b)$ What values are imbibed by Vividha?

$(a)$ Give two examples of shellfishes.
$(b)$ Mention the names of four marine fish of high economic value.