a、b、c三种元素的原子序数均小于20，a、b两元素的阳离子和c元素的阴离子都有相同的电子层结构，a原子的半径大于b原子的半径，则三种元素的原子序数的关系是

A．a＞b＞c

B．b＞a＞c

C．c＞b＞a

D．a＞c＞b

 It seemed that the conflict________have been avoided with better communication,but they were

both unwilling to give in to each other.[ ]

A. would  

B. could 

C. must  

D. needed