C) "previously" is an adverb and should refer to a verb, but $7 is a noun - you'd need the adjective "previous" here. And really you'd want to say something like "compared to the previous requirement of $7 per year"
限定性从句和非限定性从句: 限定性定语从句对先行词起限制、确定作用,与先行词有着不可分割的联系,它的前面通常没有逗号;非限定性定语从句与它的先行词之间只有比较松散的关系,通常用逗号将它与主句隔开
A正确, 非限定定语从句which holds…就近修饰Lake Baikal; more than all the…作为20 percent of the world’s fresh water的同位语
B that引导限定性定语从句前面不用逗号隔开, 而此处去掉that从句并不影响原句的主要意思,因此应该用which引导的非限定性定语从句表明两者之间松散的逻辑关系, 但即使改为which引导的非限定性定语从句,which也只能修饰fresh water仍然意思不对; has more than 300 rivers that drain into it表达笨拙,不如A的结构简单
生产商把很多玻璃瓶换成了塑料瓶,而铝罐没怎么换; 虽然铝回收比玻璃多,但生产阶段玻璃瓶就少了很多,变成了塑料瓶,于是变成垃圾的玻璃瓶就少了,从而解释了原文矛盾,正确