GMAT Sentence Correction (SC) | Re: A recent study, published by the California Bureau of

Good catch. Now I remember it. In Thursday with Ron, Ron said that a helping verb is necessary to eliminate ambiguity. Y histou can put the helping verb before or after the subject. However, if there is a noun modifier, then the helping verb needs to precede the the subject. Thus, the official answer is wrong. It should be "...than did people who..."