该句为主从复合句。句中“where one of us would do something for the other... with another favor”为where引导的定语从句，先行词为“favor war”， where在从句中作地点状语。

译文：几年前我们开始这种“互助活动”，在活动中我们中的一个人会帮助另一个人做一些事情，如铲除他的车道上的积雪或帮助他建一个棚，下次对方会尽量用另一件事来更好地回报。

5．What can we know about the writer and Orlando?

A．They once had a conflict.

B．They signed a favorable agreement.

C．They favor shoveling snow together.

D．They are always ready to help each other.

解析：D　考查细节理解。根据第一段第三句“A few years back we started this sort of ‘favor war’， where one of us would do something for the other, such as shoveling (铲) his driveway or helping him build a shed, and next time the other would try to top it with another favor”可知，几年前我们就开始这种“互相活动”，一个人帮助另一个人之后，下一次便由对方提供帮助；据此可知，作者和Orlando总是乐意互相帮助，故D项正确。