题目

We all remember dozing off (瞌睡) during a boring class at school. A robotic teacher that monitors students’ attention levels and follows the techniques human teachers use to hold their pupils’ attention promises to end the doze, especially for students who have their lessons online. Tests indicate the robot can improve how much students remember from their lessons. Intelligent tutoring systems that use virtual (虚拟的) teachers to interact with students could play a key role in the expanding field of online education. The trouble with online courses is that it is usually imposs有BaCl2和NaCl的混合溶液a L，将它均分成两份。一份滴加稀硫酸，使Ba2+离子完全沉淀；另一份滴加AgNO3溶液，使Cl－离子完全沉淀。反应中消耗x mol H2SO4、y mol AgNO3。据此得知原混合溶液中的钠离子浓度（mol·L-1）为    （    ）    A．（y-2x）/a      B．（y-x）/a              C．（2y-2x）/a      D．（2y-4x）/a