I have been worried about using any other source than GMAC

I have been worried about using any other source than GMAC for Critical Reasoning practice material. Here is a good example why:

I came across two questions that seem to contradict each other in their answer explanations, one from PowerScore CR Bible and the other from MGMAT.
Here are the questions:

PowerScore
#2 page 95 Must Be True Question Problem Set
Last year, the government of country A imposed large tariffs on steel imports in an effort to aid its domestic steel industry. Many domestic steel producers enjoyed record profits as a result, as foregin steel producers were in many cases unable to compete effectively under the burden of the newly imposed tarriffs.

Which of thhe following conclusions is best supported by the passage?
A. Not all steel producers were unaffected by country A's newly imposed tariffs.
B. Some foreign steel producers were able to compete effectively in country A even after the new tariffs were imposed.
C. After the new tariffs were imposed, most foreign steel producers were unable to compete effectively with country A's domestice steel producers.
D. Most domestic steel producers were able to increase their profits adfter the new tariffs were imposed.
E. If a government intends to protect a domestic industry, the imposition of tariffs on imports is generally an effective approach.


And here are the answer explanations from PowerScore:
(A) This is the correct answer. Since we know that domestic steel producers benefited positively from the tariffs, and foreign steel producers were affected negatively, it must be true that all steel producers were unaffected. Worded another way, answer choice A states "some steel prodcuers were affected by ...[the] tariffs."
(B) The stimulus tell us that "foreign steel producers were in many cases unable to compete effectively under the burden of the newly imposed tariffs." This does not imply that some foreign steel producers were able to compete effectively. Be careful not to assume that "some could compete" just because you are not explicitly told "all could not compete." Although this answer choice Could Be True, it does not have to be true.

---I chose not to write out the other answer explanations because I think the reason they are wrong is a little straight forward and im sure you all have the book. If you want me to I'll glady write them out.

Now for Manhattan GMAT question I recieved on a CAT. This is lableed as a 700-800 level question.

Mahnattan GMAT
Analyst: Creative professionals, such as clothing designers, graphic designers, and decorators, often have very poor managerial skills and do not succeed when they try to run their own businesses. In fact, most of these creative types are less skilled in business than is the average white-collar professional who does not work in a creative field. Generally, creative talent and business acumen rarely go hand in hand.

If the analyst's argument is taken as true, which of the following statements can properly be concluded?
A. No successful businesspeople are creative.
B. Some creative types are not less skilled at business than is the average white-collar worker who is not creative.
C. Creativity precludes success in business.
D. Any white-collar worker who is not creative is more successful in business than any creative professional.
E. Business is not a creative endeavor.

And here are the answer explanations from Manhattan GMAT:
The analyst presents several points about the business talents of creative professionals. In drawing a conclusion from the analyst's argument, we must be careful to choose a provable claim, whether or not this claim pulls together all the premises. We also must avoid extending the analyst's argument or selecting statements that are too extreme. Finally, we must not allow this process to be clouded by reactions to the content of the argument; whether or not we agree with the premises, we have to find a provable conclusion.
(A) This choice takes the passage's claim that creativity and business acumen rarely go hand in hand to an extreme. The analyst does not assert that absolutely no successful people are creative.

(B) CORRECT. The passage states that most creative types are less skilled in business than the average white-collar worker who does not work in a creative field. This implies that some creative types are not less skilled than the average white-collar worker who is not creative.
(C) This choice again takes the passage's claim that creativity and business acumen rarely go hand in hand to an extreme. Creativity and business acumen are not mutually exclusive.

(D) The passage does not say that all white-collar workers are successful, nor does it say that no creative professionals are successful.

(E) The passage makes a distinction between creative talent and business acumen. This does not mean that there are no aspects of business that fall under the realm of creativity.



So although I can see how answer A is obviously the better choice in the PowerScore question, could answer choice B be correct if answer A wasn't there? My confusion also lies in the fact that it seems that MGMAT used the same reasoning to justify answer choice B (in its question) as correct whereas, PowerScore used similar reasoning to ding answer choice B in its question.

So, does "many didn't" imply that "some did?"
What should I take away from these examples? I was able to get to the correct answers but what if the question is way harder? How can I use this "many"/"some" rule.

Using PowerScore's logic would make MGMAT's explanation wrong. Right?

I really appreciate any and all help. (was I redundant there? I dont even know now. haha)