下列各句中标点符号使用正确的一句是（ ）
—Is this your pen, Sam?
—No, it's Peter's. _______ is in my pencil case.
- Wow, Your classroom is nice and big! ( )
- _______.
双链在复制时解旋，这时下列哪一对碱基从氢键连接处分开