题目

Have you asked the teacher __________ tomorrow morning?A. if we needed an umbrella             B. when and where we shall meetB. how shall we get there                 D. what should we take 答案：B 为求值，可令S=，则2S＝，因此2S-S＝， 所以＝。仿照以上推理计算出的值是_________________。