单选题：根据下面内容，回答题： By the mid-nineteenth century, mass production o

根据下面内容，回答题：
By the mid-nineteenth century, mass production of paper patterns, the appearance of the home sewing machine, and the convenience of mail order catalogs brought fashionable clothing into the American home. By the early twentieth century, home economists working in extension and outreach programs taught women how to use paper patterns to improve the fit and efficiency of new garments as well as how to update existing ones.
Teachers of home economics traditionally made home sewing a critical part of their curriculum, emphasizing self-sufficiency and resourcefulness for young women.However, with the increasing availability of mass-produced clothing in catalogs and department stores, more and more women preferred buying garments to making them. As a result, home economists shifted their attention to consumer education. Through field study, analysis, and research, they became experts on the purchase and preservation of ready-to-wear clothing for the family, offering budgeting instruction targeted at adolescent girls.
Modern home sewing made it possible for American women to transcend their economic differences and geographic locations with clothing that was increasingly standardized.The democratization of fashion continued through the twentieth century as the ready-to-wear market expanded and home sewing became more of a pastime than a necessity.

According to the passage, the appearance of mail order catalogs altered the role of home economists because________. A．mass-produced clothing rendered their jobs completely useless
B．women ceased sewing so home economists had to teach other subjects
C．their focus shifted to instruction on budgeting and buying and preserving clothing
D．the emphasis on field study and research increased for students in home economics courses