"The price of books for our students is just getting higher and higher, and along with the rising cost of tuition（学费）, it's killing these students," said Peter Jason, a college professor. "Remember, students are one of the poorest groups of people in America. Almost half of them have at least one part-time job. In fact, one of my students has three jobs. And she still manages to have high scores and go to school full-time."
Textbook prices are usually high. Adding to that problem, many college teachers change textbooks year after year. They either upgrade to a new edition（版本）or change to a different textbook. This further hurts students because if a teacher no longer uses a textbook, that book has no resale value.
Dr. Jason decided to make life a little easier and a lot cheaper for his students by writing his own book on public speaking. "Many books have an increased price because of CD-ROMs, lots of color photographs and pictures. I talked to my students, and many of them, like me, prefer to keep things simple. So, a few years ago, I wrote my own textbook. I made sure that it was simple. Compared to most of other public speaking books, mine is half the number of pages, and one-third the price. That is $30 instead of $90. When I wrote a second edition last year, students only had to buy the 35 new pages. For only $7, they had almost a new book. "
How much does Peter Jason's second edition book usually cost?（ ）
根据文章最后一句话，第二版书，同学们只需花7dollars就可获得一本新书，故答案为A。
把10gMgCl2和MgSO4的混合物溶于适量水中，滴加2mol／L的BaCl2溶液至沉淀完全，共消耗BaCl2溶液25mL。求混合物中MgCl2和MgSO4的质量各是多少克
固定在匀强磁场中的正方形导线框abcd，各边长为1．其中ab是一段电阻为R的均匀电阻丝，其余3边均为电阻可忽略的铜线。磁场的磁感应强度为B，方向垂直纸面向里。现有一与ab段的材料、粗细、长度都相同的电阻丝PQ架在导体框上，如图所示。PQ以恒定速度方向如何
一小物块以速度v0=10m／s沿光滑地面滑行，然后沿光滑曲面上升到顶部水平的高台上，并由高台上飞出，如图所示。问当高台的高度h多大时，小物块飞行的水平距离最大这个距离是多少(g=10m／s2)
某晶体盐的水溶液呈浅绿色，在此溶液中通入氯气，溶液变为棕黄色，把所得棕黄色溶液分成两份：一份加入KSCN溶液，溶液变深红色；另一份加入BaCl2溶液，产生不溶于稀HNO3的白色沉淀，则该晶体盐的化学式是_______。
欲除去乙烷中的乙烯，可将混合气体通过盛有_______的洗气瓶；欲除去乙炔中的硫化氢，可将混合气体通过盛有_______的洗气瓶。
浓硝酸常盛装在棕色瓶中，且贮放在冷暗处，原因是_______，
碳酸氢钠溶液和烧碱溶液反应的离子方程式为_______。
常温下，氨水中存在的离子有_______。
在酸或碱存在的条件下，甲酸甲酯跟水发生水解反应的化学方程式为_______。
有下列几种气体：H2、O2、HCl、NO、NH3、CH4，其中通常情况下能用来和水做“喷泉”实验的是_______。
