GMAT Sentence Correction (SC) | Re: Many of today’s mathematicians use computers to test cases

Many of today’s mathematicians use computers to test cases thatare either too time-consuming or involve too many variables to test manually, allowing the exploration of theoretical issues that were impossible to test a generationago

a) are either too time-consuming or involve too many variables to test manually, allowing the exploration of
Either ... or construction is a parallel marker. 'Or' is followed by a verb and hence, 'either' too should. Because the helping verb 'are' is used before