Explanation:
In Bootstrap, wells are used to create a visually distinct container for content. Here's the breakdown of the statements:
(A) By default, the size of wells is medium: Yes, the default size of a well is medium. It provides a moderate padding and border-radius, giving it a "boxed" appearance.
(B) Using .well-sm and .well-lg classes, we can change the size of wells: This is correct. You can use .well-sm for small wells and .well-lg for large wells to adjust the size of the wells as needed.
(C) The .well-sm class is for small wells, and the .well-lg class is for large wells. And both classes should be used in conjunction with .well class: This is true. The .well-sm and .well-lg classes are modifiers that work together with the .well class to adjust the size of the well.
So, all of the statements are correct, making (D) All of the above the right answer.