A:BO=DO
B:S△COD=S△AOD
C:∠BAD=∠BCD
D:AC=BD
——Why do you get up so early in the morning?—— I make it a ______ to get up before 7am to read English.
口袋中装有大小质地都相同、编号为1,2,3,4,5,6的球各一只.现从中一次性随机地取出两个球,设取出的两球中较小的编号为X,则随机变量X的数学期望是____