妈妈买了3瓶橙汁，每瓶a元，付给销售员50元，应找回______元．
依题意有：50-3a（元）．
答：应找回（50-3a）元．
故答案为：（50-3a）．
Black broke his leg. He has just had _______ X-ray examination. _______ doctor said
he needed _______ operation.[ ]
A. a; The; an
B. a; A; an
C. an; A; an
D. an; The; an
停车后在微正压下保护变换催化剂的目的是什么？
