import math


def angle2D_between(c_1, c_2):
    """
    Returns the angle in radians between vectors 'vec_1' and 'vec_2'::

    """
    dx = c_2[0] - c_1[0]
    dy = c_2[1] - c_1[1]

    angle = math.atan2(dy, dx)

    return angle