一种元素的离子变成原子,这种离子发生的变化是[ ]
A.一定失去电子
B.可能失去电子,也可能得到电子
C.一定得到电子
D.既不失去电子,也不得到电子
答案:B
________the problem, he wouldn't have committed those mistakes.
A.If he understood
B.Had he understood
C.When he had understood
D.If he would understood
们判断一个物体是在运动还是静止时,总是选取某一物体作为 ,这个物体叫做 。