题目

90. ____ the work was very difficult, they managed to finish it in time. A. Because                B. Though              C. Therefore           D. But 答案：B用适当方法解下列方程：（1）（3x-1）2=1；（2）2（x+1）2=x2-1；（3）（2x-1）2+2（2x-1）=3；（4）（y+3）（1-3y）=1+2y2．