The question states that the people who just assume that big corporations will be willing to pay more taxes imposed on them without giving much thought to it are mistaken. So, the remaining passage deals with directing to the fact that they aren't that willing which some people mistakenly think they are. They fail to consider that the big corporations often have other motives behind their actions. This is said in the second section.

So, for the first blank the answer should be B) ulterior. Besides, ulterior motive is a common phrase but we need to keep in mind that this shouldn't be the criterion alone to choose the answer. The corporations think ahead of the game. And that aspect is pointed out in the first sentence.. The other two options A) openhanded and C) miserly are not at all related with self-motivated. Also, if there act is miserly or greedy, that is not the question here. There action and the motivation is in question. Not the result.

So, maybe miserly seems a plausible option in this case it is not synonymous or related with the self-motivated sense of the sentence. Open-handed just means open to many interpretation. But it is simply not. If it had been the case the people who think that the corporations are up for tax impositions wouldn't have been wrong. For the second blank also we need to provide for the idea that as to why corporate might not cater to the idea of excess tax imposition.

Thus E) bolster will further inspire them to pay the taxes willingly and F) supersede expresses too strong of an impact to their global strength, in the sentence. Thus D) sap should be the right as answer, given the fact that it means slowly draining out their strength in global respect.

I hope this clear things up. Feel free to ask questions in case of any confusion.

Regards,
Scrat