xy > 0 both are positive or both negative
xz > 0 both are positive or both negative
if x < 0 then z < 0 and y < 0 but xyz < 0 so this doesn't work
if x > 0 then y > 0 and z > 0 and xyz > 0 so this works and x > 0

(1) When t is divided by 7, the remainder is 6 --> t=7q+6t=7q+6 --> (t+2)(t+3)=(7q+8)(7q+9)(t+2)(t+3)=(7q+8)(7q+9). Now, no need to expand and multiply all the terms, just notice that when we expand all terms but the last one, which will be 8*9=72, will have 7 as a factor and 72 yields the remainder of 2 upon division by 7. Sufficient.

(1) 4 is the integer that is closest to x+y --> 3.5

The numbers x and y are NOT integers. The value of x is closest to which integer?

(1) 4 is the integer that is closest to x+y --> 3.5

为什么比较的不是circulation? circulation前面还有lower呢，那than后面为什么不是和circulation相对应的对比对象？

1978-1988，美国的生活垃圾中，饮料瓶所占的重量比重下降了。铝和玻璃回收对这种下降起了作用。但是，虽然铝比玻璃更多的回收，在生活垃圾中玻璃瓶子所占的重量比重下降的比铝多

解释题 分析题目，出现矛盾，铝比玻璃回收多，但玻璃占重量下降比重还更大，需解决矛盾

(A)玻璃瓶同等大小比铝罐重很多；但铝回收更多，无关比较，排
(B)回收的铝罐几乎都是饮料罐，而回收的玻璃瓶很大部分不是饮料瓶；瓶子或罐子是否装饮料显然无关，排
(C)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
(D) 被买的玻璃瓶总重量比铝罐总重量增长快一点；重量增长快，并不能说明重量更多，排
(E) 很多地方，玻璃瓶在回收前按颜色分类；铝罐不分类；分不分类与回收的重量显然无关，排。

the evidence in the argument is based on where this strain of wheat has been found growing, NOW in modern times (as you can tell from the present perfect, "has been found growing"). if we're going to argue about the domestication of this wheat, in ancient times, then we need to know that the same conditions that prevail now also prevailed back then.
this is why (d) strengthens the argument. without (d), it's irrelevant where this wheat grows today.

ron神说的 and表示前后独立的句子，前后没有任何关系，而这题前后是有相关性的

the weight of glass bottles in household garbage declined by a greater percentage than the weight of aluminum cans.
A选项错在，Glass bottles are significantly heavier than aluminum cans of comparable size.一直以来是一个事实，以前是这样，现在还是这样，不能用来解释变化

(E) responding没有平行的对象;primarily位置错误,置于句首则修饰整个句子; goal和creating概念不对等
句子结构: Japanese researchers are producing … that can identify…and then respond…; the…goal is to…

(A) 第二个分句中的their在逻辑意思上应该指代researchers，与前面出现的代词they指代对象不一致; goalc和creating概念不对等; primarily修饰creating,逻辑意思错误
(B) 分号后面不是一个完整的句子;then是副词,不能连接句子,应该改为and then
(C) 正确, and then respond和can identify并列;重复the researcher消除了指代歧异;goal和to do概念对等
(D) giving没有平行的对象,且使用give a response不如直接用动词respond简洁有效;goal和creation概念不对等,“目标”只能是去做某事,而不可能是一个名词“creation”

that做非限制性定语从句结构错误

if its pursuer is not cowed by the performance, will fall over and play dead. 主句的it省略了

后面一句的结论说碎片大到能够经受大气层磨损而不被磨损光后通过大气层。

而前面一句是支持结论的证据，因为只有大气层被穿透过，大气层下面的云层才会在穿透时候有机会进入大气层，从而把大气层里没有而云层里面有的硫元素带入到大气层里面。

理解不对“We are not told in what way, if at all, this discovery illuminates historical understanding.”

belong in 意为“应该归入；原住在”，属“belong+位置介词”结构，in后一般接地方或位置名词，有时根据所述的地点不同，可使用不同的介词(如belong on，belong under，belong among，etc.)

d里even though是不是也不可以和but同时出现

请问为什么similar to here is wordy and imprecise?

对题目的理解有误，对题目中的conclusion句子判断有误

但是在平行结构中，and 前不是一定不能加逗号，有时候可以作为停顿，这种情况怎么看呢？