import random


def random_sources(xSize, ySize, zSize, number):
    """ returns a list of random positions in the grid where the sources of nutrients (blood vessels) will be """
    src = []
    for _ in range(number):
        x = random.randint(0, xSize-1)
        y = random.randint(0, ySize-1)
        z = random.randint(0, zSize-1)
        if (x, y, z) not in src:
            src.append((x,y,z))
    return src