在如图所示的电路中，定值电阻的阻值R₀和电源电压U均为已知．在a、b间接入一个未知电阻R_x，闭合开关，电压表的示数为U_x，则由已知量和测得量可以推出：R_x=____，R_x的电功率P_x=____

    My dad works from Monday to Friday in a bank. He uses the computer to count money. His job is very important in the bank.

    Dad is also busy at home. At weekends he cooks dinner. Usually he cooks Italian food. On Sundays he makes five pieces of pizza. Sometimes he cooks chicken and makes Chinese food. My mum watches and helps him. I help my dad, too. I wash the dishes.

    Many people think it is strange for a man to cook. But my dad enjoys his hobby. Cooking relaxes him. He is a weekend cook.

下列物质属于纯净物的是(　　)。