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．
Who shared with the class a rock that floats
A.Lisa
B.Kara
C.Justin
D.Mark