Nidhs wrote:I understand that b does not mantain parallelism...but can someone please explain to me why the verbs add and combine are in simple present and not present progressive.

there's a difference between the time frame of the progressive (is undergoing) and the time frames of the events 'add' and 'combine'.

the former is an ongoing process: gasoline marketing is undergoing major changes. if you were to plot this process on a number line that represents time, it would be a line segment or a ray (depending on whether it ends); it would not be a single point event.

the latter, though, are one-off events: individual gas stations add convenience stores, and individual gas stations combine with fast-food chains. if you were to plot these events on your number line, they'd be single points. the progressive tense is therefore inappropriate for these events, because they aren't ongoing. (the trend formed by ALL of the one-off events is ongoing, but that's exactly what is implied by the use of the progressive tense in the sentence.)

hope that helps.