题目内容:
Questions 21—30·Read the article below about improvements.
·Choose the correct word or phrase to fill each gap from A, B, C, or D.
·For each question 21—30, mark one letter (A, B, C, or D) on your Answer Sheet.
·There is an example at the beginning, (0).

请根据以上资料回答第21题__________

参考答案:
答案解析: