work out the number of sub-atomic particles in an atom of
A. beryllium
B. phosphorus
C. nitrogen
D. potassium

答案：A.4p,4e,5n B. 15p,15e,16n, C.7p,7e,7n
D19p,19e,20n
怎么做出来的？请帮忙解释一下，详细步骤.谢谢