20. I received this letter from Japan two weeks ago.
The sentence is correct, so no correction is needed.

21. My father often doesn’t travel to work by car.
The error is with the word "doesn't." The correct sentence should be:
My father doesn't often travel to work by car.
The underlined part that needs correction is "often doesn't".
Correct answer: B.

22. The workers is building a new house right now.
The error is in the verb agreement. "Workers" is plural, so the correct form should be "are."
The correct sentence is:
The workers are building a new house right now.
The underlined part that needs correction is "is".
Correct answer: C.

23. Because the weather was bad, we still went to the beach with our friends.
The error is in the use of "still." The word "still" implies an action happened despite some condition, but the sentence doesn't clearly convey that meaning. A more natural way would be:
Because the weather was bad, we went to the beach with our friends anyway.
The underlined part that needs correction is "still".
Correct answer: D.

20. A. receive -> received
21. B. often doesn’t -> doesn’t often
22. A. is -> are
23. A. Because -> Although
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