If said there were about 500, 000 visitors flocking to the city for the 3rd International Aviation and Aerospace Exhibition, that would be a ______ estimate—in fact there are probably a lot more.
 
【正确答案】 B
【答案解析】 固定搭配。conservative estimate意为“保守的估计”，为常用搭配。故答案为B。affirmative“肯定的，乐观的”；progressive“前进的，进步的”。