Sentence Correction

Gasoline marketing is undergoing major changes as stations often not only add convenience stores but also combine with major fast-food chains to build complexes where customers can shop and eat as well as buy gasoline.
a) same
b) are not adding convenience stores, but also in combining with major fast-food chains they build

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.

Nidhs wrote:Gasoline marketing is undergoing major changes as stations often not only add convenience stores but also combine with major fast-food chains to build complexes where customers can shop and eat as well as buy gasoline.
a) same
b) are not adding convenience stores, but also in combining with major fast-food chains they build

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.

The culprit is the word "often".

Often means that we're referring to a number of individual events, rather than 1 continuous event.

For example, we would say "On Sundays, I often go for a drive" rather than "On Sundays, I am often going for a drive".