题干
—This history book is of great value.
—Exactly. _____ can be enjoyed from it ______ you have a deep understanding of it, however.
A：Nothing; unless
B：Few; till
C：Something; until
D：Little; since
答案(点此获取答案解析）
A
的焦点为
，准线为
，
为抛物线上一点，
为垂足. 如果直线
的斜率为
，那么
（ ）
