题目

20.在如图所示的电路中，R1、R2、R3和R4皆为定值电阻，R5为可变电阻，电源的电动势为ε，内阻为r。设电流表A的读数为I，电压表V的读数为U。当R5的滑动触点向图中a端移动时……………………（ ）A.I变大，U变小                                  B.I变大，U变大C.I变小，U变大                                  D.I变小，U变小I bet Mrs. Black will come to help us with the celebration if she ______ too busy tomorrow. [     ]A. is B. will be C. won't be D. isn't