def choose_currency():
    """Gets user input on their preferred currency."""
    print('At the moment, we only support trading a single currency at a time.'
        )
    print('Here are your choices:')
    print('[1] Bitcoin')
    print('[2] Ethereum')
    print('[3] Litecoin')
    while True:
        currency = input('Which do you choose? ')
        if currency == '1':
            return 'Currency=BTC\n'
        elif currency == '2':
            return 'Currency=ETH\n'
        elif currency == '3':
            return 'Currency=LTC\n'
        else:
            print('Please enter in a valid number.')
