Here are the correct answers for the blanks in the sentences:

16. By the end of last year, George **A. had learned** English for two years.
17. She **A. painted** the picture when I came.
18. While I **B. was sweeping** the floor, Mrs. Parker began cooking.
19. She **C. has washed** dishes already.

If you need any further explanations or assistance, feel free to ask!