第30题: [单项选择]They concentrated their efforts on ______ products of high quality. A. turning on B. turning off C. turning out D. turning over 参考答案:C 答案解析:本题测试的是动词短语的辨析。全句的意思是:“他们集中精力生产高质量的产品。”turn sth. on意为“打开”;rum sth. off意为“关闭”;turn sth. out意为“制造,生产”;t