题目

       Dick lived in England. One day in January he said to his wife, I'm going to fly to New York next week because I've got some work there. Where are you going to stay there? his wife asked. I don't know yet. Dick answered. Please send me your address from there in a telegram (电报), his wife said. All right, Dick answered.       He flew to New York on January 31st and found a nice hotel in the center of the city. He put his things in his room and then he sent his wife a telegram. He put the address of his hotel in it.     In the evening he didn't have any work, so h16．式子x2y，$-\frac{8}{7}mn$，$\frac{1}{2}$，3s-1，0，m，$\frac{a+b}{5}$，$\frac{y}{x}$中单项式有（　　）A．4个B．6个C．5个D．7个