18.What is missing data? 19. Why is missing data filled in Dataframe with some value? 20. Name the function you can use for filling missing data? 21. Name some function to handle missing data.

Ans:
18. NA values such as NULL or NaN or None values which can not participate in any computation are called missing values.
19. To calculate accurate result or predictions from given data, missing data should be handled or filled.
20. fillna(<n>)
21. dropna(), fillna(), isnull()