坡底线
参考答案：
台阶下部平盘与坡面的交线。
Since you have practiced so much in driving, there ______ be any difficulty in passing the road rest. [ ]
A. mustn't
B. shouldn't
C. shan't
D. needn't
设
(其中a,b,c,d为常数且a≠0,c≠0)，则一定有 ( )
(其中a,b,c,d为常数且a≠0,c≠0)，则一定有 ( )