Here, some answer choices contain "competing companies". Others contain "companies that compete", switching those two elements.
This distinction should call your attention to the neighboring phrase "one day" which can only sensibly describe competition, not the companies themselves.
in industries that are more established & in the more established industries

AB they refers to ancient anasazi

A真正的错误：cost to sb表示承受cost的人，cost of doing表示造成cost的事。排除AE
C选项：RON说by the end of the decade修饰 is projected，doesn't make sense。according to the modifier, then, the PROJECTION will occur by the end of the decade. that's not the intended meaning (the sentence is clearly meant to say that the projection is being made right now, but that the cost will be at level X by the end of the decade).
D修饰的逻辑错误同C

条件2：任意两个数之和为0

抛物线啥的太烦了。
就(y2-y1)/y1比大小好了。
81/2500>73/24919>65/28846>55/24781>49/24724
分子分母同时减小的，看分子减小的幅度大还是分母减小的幅度大判断分数是变大还是变小了。

设每箱有n加仑。462/n-336/n=6, n=21
每加仑的公里数=每箱的公里数/每箱的加仑数=336/21=16

"However, anthropologist Shepard Krech points out that large animal species vanished even in areas where there is no evidence to demonstrate that Paleoindians hunted them. Nor were extinctions confined to large animals: small animals, plants, and insects disappeared, presumably not all through human consumption."
C 气候变化不仅发生在P末期还发生在其他时代，未说明其他时代有发生类似的灭绝。

(n - k) * (n + k) = 48
计算48可以分解成多少个2数相乘的组合，48 因数分解：(2,2,2,2,3),可以分解成多少个两数相乘
(2，24), (4，12), (8，6), (16，3)
小的数=n-k，大的数=n+k，因为n-k+n+k=2n,n是整数，所以两数的和只能是偶数,去掉(16，3) 的组合，剩下3组。

居然没有发现B里的AGREEMENT!!!

D the rate is more than 50% NOT half.

首先搞懂题目问题，剩余要有多少比例投反对票才能使得总票数有2/3的反对票
2/9 * 1/4 +7/9 * X = 2/3

看到multiple,第一步要写成质数的形式：15=3*5
1）n=20a=2²*5*a，当a是3的倍数时，n才是15的倍数，不充分
2）n+6=3b，得n=3*（b-2），当b-2是5的倍数时，n才是15的倍数，不充分
1）+2）：n的因子既有5又有3，必然是15的倍数，充分

看到multiple,第一步要写成质数的形式：15=3*5
1）n=20a=2²*5*a，当a是3的倍数时，n才是15的倍数，不充分
2）n+6=3b，得n=3*（b-2），当b-2是5的倍数时，n才是15的倍数，不充分
1）+2）：n的因子既有5又有3，必然是15的倍数，充分

cr
共同之处 Testing with以及citing a cardiac enzyme blood test.，重点都是test

A,B,C的比较都是对的；don't forget—the OG answer keys are usually incomplete, and often incorrect.
the official PROBLEMS are essentially flawless, but the official answer keys are not. (GMAC's all-stars, the people who write the problems, do not write the answer keys; GMAC 'outsources' that work to people of lesser talent, presumably to cut costs.)

a选项虽然有点别扭，但是b的it指代问题错的太明显了！！！！

先排4个不同的，再把2个相同的塞到4个排好的字母的间隔里，因为是塞两个相同的字母，所以不分顺序，是C
A(4,4)*C(5,2)=240

网格路径问题
无论路径如何，从X到Y都需要经过3条street和2条avenue，相当于从5个里面挑3个street，street确定了，avenue自然就确定了。

disinclination/disinclined 不情愿 是不能用have+n形式来等价的
我们可以说：
Jack hits the stone. （杰克打石头）
但不能说：
**Jack has the hitting of the stone. （**杰克拥有石头的打）