在营养物质的四种运输方式中，只有（）运输方式改变了被运输物质的化学组成。

假设你是Tom，根据下面的表格内容，给你的笔友Jim写信描述你上周周末所作的事，并询问他的周末。要求60词左右。开头结尾已经给出。

Saturday morning	Go shopping with friends
Saturday afternoon	Clean the room, cook dinner
Saturday night	Watch TV with parents
Sunday morning	Play basketball with friends
Sunday afternoon	Stay at home , practice English
Sunday evening	Do homework

Dear Jim,
How’s it going? I had an interesting weekend. _________________________________________
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Can you write and tell me about your weekend?
Yours，
Tom