题目

At 7:35 p.m. March 28 To: The manager Clothes Horse Store Dear Madam/Sir,         I am writing to tell you about an annoying experience I have just had in your store.         Yesterday morning I bought a shirt from your store. The product was a red shirt, size 12, prize $140. When I returned home and took it out of the bag, the sleeve was almost completely off. I had traveled home in the train with the bag and treated it carefully.         I went back to the store in the afternoon to return the shirt and exchange it for another. The shop assistant refused to exchange the s19．sin240°+sin220°+sin40°•sin20°的值为（　　）A．$\frac{3}{4}$B．$\frac{1}{2}$C．$\frac{1}{3}$D．$\frac{1}{4}$