Sentence Correction

Hi, while going through Manhattan GMAT SC material, I found below example:

Tabacco companies, shaken by a string of legal setbacks in the United States, but which retain strong growth prospects in the developing world, face an uncertain future.

Parallel Marker: But
Parallel Elements: Relative Pronoun
Shaken by -> Which have been shaken by
Which retain strong growth

As per solution it says "shaken" should be "which have been shaken".

I thought BEEN should be used in present continuous tense But here "been" is made parallel to present tense(which retain strong growth).

Can you please explain me the use of "been" in detail. I searched in google but i didnt get proper answer..

Tobacco companies, WHICH HAVE BEEN SHAKEN by a string of legal setbacks in the United States, but which RETAIN strong growth prospects in the developing world, FACE an uncertain future.

Have + been + VERBed = the passive voice of the present perfect.
One purpose of the present perfect is to express a past action that AFFECTS THE PRESENT.
Here, a past action (the companies HAVE BEEN SHAKEN) affects a present action (why the companies FACE an uncertain future).
RETAIN is in the simple present tense because this action is happening right now, at the same time as the companies FACE an uncertain future.

BTG14 wrote: I thought BEEN should be used in present continuous tense But here "been" is made parallel to present tense(which retain strong growth).

Parallelism does not mean that the entire sentence needs to be in the same tense. For example, following sentence is correct:

James struggled in the past, but is a very successful businessman now.

First part of the sentence is Simple Past, second part is Simple Present.

Hi,

Is the construction BUT WHICH is correct in GMAT ?

I am saying this because i have read somewhere for the explanation of the Option B of Ques no. 106 of OG 12/SC that BUT WHICH construction is always wrong in GMAT.

Pls throw some insightful regarding this.

THANKS