You may borrow this book______you keep it clean.

选A as long as 表示 只要 的意思.只要你能把书保持的干干净净,你就可以借走这本书.当然as long as 字面意思是长达.比如,the great wall is as long as 10000km.长城长达一万米.而as far as 字面意思是远达.,比如Beijing is as far as 1200000m from my homtown.另一层含义是据.常常构成as far as i know 据我所知 .
当然了因为两者是as..as...结构,所以本身意思还有和,一样远或者长的意思哦.
in case 以免...
even if 即使
我解释的还不够好吗?楼主?

选A as long as 只要
as far as 就....来说

选A as long as 只要
as far as 就....两者是as。。as。。。结构，所以本身意思还有和，，，，一样远或者长的意思哦。

选择A。“只要你能保持书的整洁，你可以借这本书”。
as long as 主要是指时间上的，而as far as 主要是指距离上的，例如“远达。。。。远到。。。。”。

as far as 没有只要的意思
as far as
Also, so far as. To the extent, degree, or amount that. This phrase alone is always used to modify a verb, as in As far as I can tell it's an authentic antique, or ...