在怀孕期间，胎儿和母亲之间进行物质交换的结构是（    ）

学生会要表彰一批优秀学生，Tom想推荐他的同学John竞争最乐于助人奖(Most Helpful Student Award)。请以Tom的名义给学生会主席写一封推荐信。词数约80，不包括已给出的开头和结尾。
推荐信中必须包括下列要点：John
1、有礼貌，勤奋，乐于助人，常帮助同学学习
2、聪明，记忆力好，学东西很快
3、经常为希望工程募集物品，照顾老年之家的老人
4、电脑用得好，经常为帮扶俱乐部(the Helping Hands Club)从网上获取资料
5、上周，一个小男孩迷路，John将他送回家，男孩的父母很感激他。
Dear Sir
I would like                                                              
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
                                                                        
We will be glad if he can win the award.
Thank you
Yours faithfully
Tom
Best wishes for you!  Hope to see you soon!