某三种短周期元素的核内质子数之和为31,它们原子的最外层电子数之和是17,则这三种元素可能是()
A、N;O;Cl
B、O;F;Si
C、N;P;Cl
D、O;P;S
参考答案:B
He was _______ teaching _______ English.[ ]
A. already; him
B. always; himself
C. often; him
D. always; his
打开排风扇的目的是什么?