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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def source_url(farm, server, id, secret, size):
    ''' Url for direct jpg use. '''
    if size == 'small':
        img_size = 'n'
    elif size == 'medium':
        img_size = 'c'
    elif size == 'large':
        img_size = 'b'

    return 'https://farm{}.staticflickr.com/{}/{}_{}_{}.jpg'.format(
        farm, server, id, secret, img_size)