Mark the letter A, B, C, or D on your answer sheet to indicate the cor

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions

The air quality in this area has improved a lot since that factory _____.

• A. was closing
• B. would close
• C. closed
• D. will close

Trong thì hiện tại hoàn thành, mệnh đề sau “since” là thì quá khứ đơn