题目

 If he had spent more time practising speaking English before, he       able to speak it much better now. A. will be           B. would be          C. has been              D. would have been 答案：B在离子RO3n-中，共有x个核外电子，R原子的质量数为A，氧原子的质子数为8，则R原子核内含有的质子数目是（　　）A．A-x+n+48B．A-x+n+24C．x-n-24D．X+n-24