问题详情

Accordingtothepassage，whathappenstohomingpigeonswhentheyareaboutamonthold？查看材料
A.Theyarekeptinatrap．
B.Theyentertheirfirstrace．
C.Theybeginatrainingprogram．
D.Theygettheirwingsclippedandmarked．
请帮忙给出正确答案和分析，谢谢！