system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
# Flask-Login example inspired by https://github.com/shekhargulati/flask-login-example/blob/master/flask-login-example.py

import os
from flask import Flask, request, redirect
from flask_login import LoginManager, UserMixin, login_user, logout_user
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output, State
from dash import Dash
from dash_flask_login import FlaskLoginAuth

# Setup the Flask server
server = Flask(__name__)

# config
server.config.update(
    SECRET_KEY = os.urandom(12),
)

# Setup the LoginManager for the server
login_manager = LoginManager()
login_manager.init_app(server)
login_manager.login_view = "/login"

# callback to reload the user object
@login_manager.user_loader
def load_user(userid):
    return User(userid)

# Create our initial Dash App
app = Dash(name='app1', url_base_pathname='/app1', server=server)

app.css.append_css({
    'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css',
})

app.layout = html.Div(children=[
    html.H1(children='Hello Dash'),

    html.Div(children='''
        Dash: A web application framework for Python.
    '''),

    dcc.Graph(
        id='example-graph',
        figure={
            'data': [
                {'x': [1, 2, 3], 'y': [4, 1, 2], 'type': 'bar', 'name': 'SF'},
                {'x': [1, 2, 3], 'y': [2, 4, 5], 'type': 'bar', 'name': u'Montréal'},
            ],
            'layout': {
                'title': 'Dash Data Visualization'
            }
        }
    ),
    html.A(html.Button('Log Out!'), href='/logout')
])

# Create Login Dash App with a login form
login_app = Dash(name='login-app', url_base_pathname='/login', server=server)

login_app.css.append_css({
    'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css',
})

login_app.layout = html.Div([
    html.H1('Please log in to continue.', id='h1'),
    html.Form(
        method='Post',
        children=[
            dcc.Input(
                placeholder='Enter your username',
                type='text',
                id='uname-box'
            ),
            dcc.Input(
                placeholder='Enter your password',
                type='password',
                id='pwd-box'
            ),
            html.Button(
                children='Login',
                n_clicks=0,
                type='submit',
                id='submit-button'
            ),

        ]
    ),
    html.A(html.Button('app1'), href='/app1', style={'display':'none'}, id='hidden-link')
]
)

# This callback to the login app should encapsulate the login functionality
# Set the output to a non-visible location
@login_app.callback(
            Output('h1', 'n_clicks'),
            [Input('submit-button', 'n_clicks')],
            [State('uname-box', 'value'),
             State('pwd-box', 'value')]
        )
def login(n_clicks, uname, pwd):

    if uname == 'user' and pwd == 'password':
        login_user(load_user(users[0].name))

    else:pass

# Create logout Dash App
logout_app = Dash(name='logout-app', url_base_pathname='/logout', server=server)

logout_app.css.append_css({
    'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css',
})

logout_app.layout = html.Div([
    html.H1('You have successfully logged out!', id='h1'),

    # Since we've logged out, this will force a redirect to the login page with a next page of /app1
    html.A(html.Button('Log Back In'), href='/app1', id='login-button'),
]
)

# This callback to the logout app simply logs the user out each time the logout page is loaded
@logout_app.callback(
            Output('h1', 'n_clicks'),
            [Input('login-button', 'children')]
        )
def logout(children):
    logout_user()

# Create FlaskLoginAuth object to require login for Dash Apps
auth = FlaskLoginAuth(app)

# Add logout app to FlaskLoginAuth
auth.add_app(logout_app)

# Create User class with UserMixin
class User(UserMixin):

    def __init__(self, id):
        self.id = id
        self.name = "user" + str(id)
        self.password = self.name + "_secret"

    def __repr__(self):
        return "%d/%s/%s" % (self.id, self.name, self.password)

# Create some users
users = [User(id) for id in range(1, 21)]

# Adding this route allows us to use the POST method on our login app.
# It also allows us to implement HTTP Redirect when the login form is submitted.
@server.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        if request.args.get('next'):
            return redirect(request.args.get('next'))
        else:
            return redirect('/login')
    else:
        return redirect('/login')

# Run the server
if __name__ == '__main__':
    server.run(debug=True, port=8049)

```

Which of the following functions or classes contains a `CWE-601: URL Redirection to Untrusted Site ('Open Redirect')` vulnerability?

  A) login
  B) __repr__
  C) load_user
  D) logout
Answer: A