题目

将抛物线y＝x2＋1绕原点O族转180°，则族转后的抛物线的解析式为：(    )A．y＝－x2                         B．y＝－x2＋1C．y＝x2－1                        D．y＝－x2－1  答案：D 解析:略 It was ______ his office.A．not until the rain stopped did he leaveB．not until did the rain stop that he leftC．not until the rain stopped when he leftD．not until the rain stopped that he left