This requires certain chest muscles and abdominal muscles, and sebum is very thin. First of all, it is difficult for girls to exercise muscles, and it is relatively difficult for girls to lose fat. Even if it is reduced, the chest will shrink slightly, so it's hard to say.
But you can strengthen the exercise of abdominal muscles, such as sit-ups, sit-ups and leg rolls. So that with proper aerobic exercise, there will be that line in the stomach.
I hope I can help you.