What is the main variable that gets manipulated (changed) in Bertrand and Mullainathan’s experiment?

Answer Choices:
Location
Gender
Race
Socioeconomic Status

in the short story the slip-over sweater by jesse stuart, the main character and narrator shan is friends with grace, an intelligent girl he has known since childhood and with whom he used to be in love.

at the beginning of the story, shan believes he’s no longer in love with grace, but loves jo-anne instead. in order to buy a slip-over sweater and have jo-anne wear it so that the other students would know she loved him too, shan takes a loan at the bank.

while jo-anne is wearing shan’s sweater, his friend grace starts to wear another popular boy’s one, roy’s sweater. because of that, the other students begin to pay attention to grace and compliment her perseverance and effort. they admire her for walking 5 miles to school every day, despite the weather, and still getting the best grades. shan, who knows all of that for a fact, says, as the narrator, ‘and i knew she had the durability and the toughness of a storm-battered mountain oak.’ he compares his friend’s strength – not only physical, but also of character – to an oak tree, whose wood is known for being sturdy.

as the story goes on, shan changes his behavior out of concern for the debt he has to pay at the bank. his new girlfriend jo-anne never asks him what is wrong nor offers any sort of . instead, she breaks up with shan for not being as fun as she thought he was supposed to be. grace, however, not only asks about shan’s problem, but him make the money he needs. shan can’t but notice how different the girls are and what a wonderful person grace is.   even though he used to find jo-anne prettier at the beginning of the story, he now says, ‘grace was as beautiful as our mountain was in april, prettier than a blossom of wild phlox or a mountain daisy.’ his comparing grace to a mountain or something that exists on a mountain (such as a mountain oak or daisy) certainly has important meaning since shan loves the mountain and everything about it and he knows the mountain thoroughly. shan also loves and knows grace as completely as he does the mountain.

i believe the correct answer is a