import random


def roulette(old_pool):
    """ Roulette wheel selection of a chromosome in the old_pool.
    Returns only the chromosome (list of strings).
    Old_pool is expected to be a list of 2-element tuples,
    where each tuple is: (chromosome, fitness_value);
    where chromosome is a list of binary strings, and
    fitness_value is its fitness value. """
    total = sum(f for (c, f) in old_pool)
    selection = random.uniform(0, total)
    spinner = 0
    for c, f in old_pool:
        spinner += f
        if spinner >= selection:
            return c