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
首先,明确什么是500 is the multiple of 100 that is closest to x。There are lots of multiples of 100,eg 100, 200, 300, 400, 500, 600 etc, but 500 is THE ONE which is closest to X.
题干:已知450
。。。