The oldest man had just celebrated his hundredth birthday and the reporter of a local paper called on him for an interview .Having congratulated the old man, the reporter asked a few questions. “
11. Though I have often heard this song ______, I have never heard you ______ it. A. being sung; sang B. sang; singing C. sung; sing D. to be sung; to sing
（a为常数）的图象上有三点（x1，y1）、（x2，y2）、（x3，y3），且x1＜x2＜0＜x3，则y1、y2、y3的大小关系是 。