In Ji' nan there are _     ___ people.    [ ]

A. about six millions    

B. millions of    

C. million of          

D. six million of

对正整数a，b，定义a△b等于由a开始的连续b个正整数之和，如：2△3=2+3+4，又如：5△4=5+6+7+8=26．若1△x=15，求x．