《最新GMAT数学机考模拟150题》陈向东__043 请指教谢谢

Jerome
wrote each of the integers 1 through 20, inclusive, on a separate index
card. He placed the cards in a box, and then drew cards one at a time
randomly from the box, without returning the cards he had already drawn
to the box. In order to ensure that the sum of all the cards he drew
was even, how many card did Jerome have to draw?

A.19      B.12         C.11        D.10          E.3

正确答案是：B

    请告诉小弟我解题的详细过程，谢谢了……万分感谢……