A:﹣2b3+3ab2+4a2b+a3
B:a3+4a2b+3ab2﹣2b3
C:4a2b+3ab2﹣2b3+a3
D:4a2b+3ab2+a3﹣2b3
设△ABC的内角A,B,C所对边的长分别为a,b,c若B=2A,b=3a,则角A= ( )