若三角形的底边长为2a+1,高为2a-1,则此三角形的面积为（　　）

_______ originally as the biggest defense work in ancient China, the Great Wall today has become one of the must–see places for visiting foreign leaders to the country in the past six decades. Obama is the fifth US president _______ the man-made wonder.

Oh, I’m not feeling well in the stomach .I       so much fried chicken just now.

____，无语东流。（柳永《八声甘州》）

—— _______ is the train station from the school?

——About twenty-minute bike ride.