引用他人的解释。
可以举几个数字来说明一下这道题
例如, 假设测试的人数是1000人,
假如该1000人是没使用咖啡因的, 根据文章, 得知可能有5%即50人呈阳性却没使用咖啡因;
假如该1000人中有5%确实使用了咖啡因, 根据题意, 使用了咖啡因的99%会被检出, 则约有50人呈阳性的人使用了咖啡因.
因此, 如果没告诉测试人群中使用咖啡因的比例, 测试结果大大不同. 文中结论说呈阳性则使用了咖啡因是不正确的.
the vast people is...,the vast...will be 意思是测试的人越多, 越......, the more ...the more 意思.
CR will get more interesting when math is involved. The most seen math problem in GMAT CR is percentage. What ETS wants to test is whether you can distinguish between absolute value and relative value, e.g. percentage.
What you need to remember is that higher percentage in CR does not indicate that the absolution number or quantity will be higher compared to lower percentage. To the contrary, higher percentage always has smaller numbers or quantity.
In this question, the conclusion is the last sentence. The premises are that 5% of non-users and 99% of users are tested positive. The argument fails to consider the composition of population and makes an unwarranted assumption that among a random group of people, non-users are less than 99/5 of users. X: non-users Y: users. 5%X < 99%Y è x<99/5Y. If this proportion is not true, the argument does not hold.
So you need to know whether the question is about absolute value or relative value. If it is about relative value, you need to know what the BASE is. If comparison is involved, you need to find out the relationship between the bases. | 
