题目

氧化镁（MgO）可用作耐高温材料，其中镁元素的化合价是 A. +1 B. +2 C. +3 D. +4 答案：B 【解析】氧化镁中，氧一般显-2价，根据化合物化合价代数和为零计算，设镁的化合价为x，则x+（-2）=0，得x=+2。 故选B。Tom was sorry to learn that his grandfather _____ for half an hour when he got to the hospital．A．had died B．had been deadC．Died D．has been dead