已知X，Y为短周期元素，两元素形成化合物XY2，已知原子序数分别为a和b，则a不可能为[ ]
A．b+4
B．b-5
C．b+6
D．b+8
答案：A
If two fair coins are flipped and two fair six - sided dice are rolled, all at the same time, what is the probability of ending up with two heads (on the coins) and two sixes (on the dice)（）
A. 0.4167.
B. 0.0039.
C. 0.0069.
男性，49岁。因头晕半天、黑便3次急诊入院。既往史无特殊。血压80/50mmHg，心率124次/分，面色苍白，伴冷汗。
该病人的诊断可能是
A．急性肠炎
B．急性胃穿孔
C．上消化道出血
D．下消化道出血
