Explain the difference in meaning between sentences A and B

1. A. Bill

            has been reading
         a book about global warming.
   

   B. Bill has read a book about global warming.

2. A. Laura

            has been painting
         the walls in her room all morning.
   

   B. Laura has painted half of her room.

3. A. How long

            have you been writing
         this essay?
   

   B. How many pages of your essay have you edited yet?

4. A. Jill is still working on the project. She

            has been trying
         to finish it all week.
   

   B. Jill has already prepared eight documents.