为了保证锚杆的托片紧贴顶板与煤帮，达到预紧力，故锚杆必须用什么机具拧紧？
参考答案：
必须用机械（锚杆机等）或力矩扳手拧紧
—Would you like to come to dinner tonight?
—I'd like to, __________I'm too busy. [ ]
A. and
B. if
C. but
D. so
旱作农业
