根据下面资料，回答43-46题
It was Sharing Day! The teacher had asked the students to bring something interesting to class．
Jimmy found the little box he and Dad had made．Inside was a piece of rock, a sand dollar and a shark (鲨鱼) tooth．Nobody could have anything as interesting as these !
In class, Jimmy waited anxiously, hoping to be called on first by the teacher．But Kara was first．
She held up a sand dollar．Jimmy's eyes grew big．
"I found this on the beach, "Kara said．"It' s interesting．The little holes that make the sand dollar look like a flower are breathing holes．See all the needles They help the sand dollar move and dig into the sand．"
Jimmy was a little uneasy．Oh, well, he still had his rock and shark tooth．
Then Mark stood up． "This is my favorite rock, called pumice (浮石)．" Then Mark dropped the rock into a glass of water．"Look ! Pumice is the only rock that floats !"
Jimmy put up his hand and waved, but the teacher called on Justin instead．
"I found this shark tooth in Hawaii last Christmas, "Justin said．
The class was excited, but Jimmy lowered his head．"Sharks have rows of teeth．Every time a shark loses a tooth, another one takes its place．"Justin passed the tooth around．
Jimmy heard the teacher call him．He stood on one foot and then the other．"Well, ah, see．．．my treasure box is interesting ! It was just a piece of wood until Dad and I cut it into pieces and finally made a box out of it．It smells good, reminding us of camping．"
"What's inside" asked Lisa．
Jimmy opened his box．"A sand dollar, a piece of pumice and a shark tooth．"
His classmates' mouths dropped open．The teacher looked at the objects in the box and smiled．
"Now that' s interesting ! "he said．
What does a sand dollar look like according to Kara
事实细节题。根据第四段可知，卡拉提到她在沙滩上找到了一个海胆，海胆上的小洞洞让海胆看起来像一朵花，这些洞洞是海胆的呼吸口，海胆上的针状物帮助海胆移动，并钻进沙子。故本题选C。
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_______。
