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
×100）来衡量其优劣，13%以上为优等品．一种制备过碳酸钠的工艺流程如图1： 