The whole idea and purpose of “higher education” is to urge students to do “critical thinking.” The choice of “$0” was deliberately left out to see if any students would object to the way the question was structured and if they would argue in their essay/discussion that it should have been an option. It did not occur to any of the students to do so.
When I took the GRE to get into grad school, one of the essay questions involved a math problem involving cows and other farm animals. I objected to the question as being “culturally insensitive” to those of us who had not grown up on a farm or knew next to nothing about animal husbandry. And so I left the question unanswered, but I explained, instead, why I didn’t answer it. I passed “with flying colors” and went on to earn a Masters in Political Science two years later. Likewise with Hacker’s “experiment.” He was trying to get the students to “think out of the box.” But their “socialization,” i.e., racism (conscious or unconscious) would not allow them to do so.