钻遇油气层执行井控坐岗制度，一般井控坐岗记录每（）分钟记录一次。
A.5
B.10
C.15
D.30
参考答案：C
方程（x-1）（x+1）=1转化为一元二次方程的一般形式是______．
______ there were only five soldiers left at the front, ______ they went on fighting. [ ]
A. Because; so
B. If; and
C. Though; but
D. Though; /
