How to change the scene naturally when writing a novel? Can I use a blank line (a whole line)? For example, a and b are doing one thing, and then they want to say b and c.

There is no need to worry about it. Blank lines are commonly used, and some authors add a horizontal line to the blank lines. If it is a story in a story, you can write "... (before), this scene seems to have always existed somewhere in your mind, yes, for A, it is ... (then recall)", and there is the structure of the article. It is too difficult to advise you not to use it in parallel (if it is not handled well, it will definitely leave defects). If it is one person and two people, your problem does not exist at all. If it is a three-person a and b perspective. Just change it directly, and there will be no unnatural feeling (it won't be embarrassing to fix the previous plot before changing it, of course, you can leave some suspense appropriately). Also, prioritize. If it's equally important, so is it. Just add some time words appropriately (after grade three, etc. )