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
from datetime import datetime

from flask import (abort, flash, redirect, render_template, request,
                   url_for)
from flask_login import (current_user, login_required, login_user,
                         logout_user)
from werkzeug.urls import url_parse

from klaverjas import app, db
from klaverjas.forms import LoginForm, RegistrationForm
from klaverjas.models import Game, User


@app.before_request
def before_request():
    if current_user.is_authenticated:
        current_user.last_seen = datetime.utcnow()
        db.session.commit()


@app.route('/favicon.ico')
def favicon():
    return redirect(url_for('static', filename='favicon.ico'))


@app.route('/')
@login_required
def index():
    return render_template('index.html')


@app.route('/games')
@login_required
def games():
    page = request.args.get('page', 1, type=int)
    games = Game.query.filter((Game.north == current_user) |
                              (Game.east == current_user) |
                              (Game.south == current_user) |
                              (Game.west == current_user)).paginate(page,
                                                                    10,
                                                                    False)
    return render_template('games.html', games=games.items)


@app.route('/games/<game_id>')
@login_required
def game(game_id):
    game = Game.query.filter_by(id=game_id).first_or_404()
    if game.has_user(current_user):
        return render_template('game.html', game=game_id)
    abort(403)


@app.route('/users/<username>')
@login_required
def user(username):
    user = User.query.filter_by(username=username).first_or_404()
    return render_template('user.html', user=user)


@app.route('/users/<username>/follow')
@login_required
def follow(username):
    user = User.query.filter_by(username=username).first()
    if user is None:
        flash('User {} not found.'.format(username))
        return redirect(url_for('index'))
    if user == current_user:
        flash('You cannot follow yourself!')
        return redirect(url_for('user', username=username))
    current_user.follow(user)
    db.session.commit()
    flash('You are following {}!'.format(username))
    return redirect(url_for('user', username=username))


@app.route('/users/<username>/unfollow')
@login_required
def unfollow(username):
    user = User.query.filter_by(username=username).first()
    if user is None:
        flash('User {} not found.'.format(username))
        return redirect(url_for('index'))
    if user == current_user:
        flash('You cannot unfollow yourself!')
        return redirect(url_for('user', username=username))
    current_user.unfollow(user)
    db.session.commit()
    flash('You are not following {}.'.format(username))
    return redirect(url_for('user', username=username))


@app.route('/login', methods=['GET', 'POST'])
def login():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = LoginForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.username.data).first()
        if user is None or not user.check_password(form.password.data):
            app.logger.info('invalid login user {}'.format(form.username.data))
            flash('Invalid username or password')
            return redirect(url_for('login'))
        app.logger.info('login user {}'.format(user.username))
        login_user(user, remember=form.remember_me.data)
        next_page = request.args.get('next')
        if not next_page or url_parse(next_page).netloc != '':
            next_page = url_for('index')
        return redirect(next_page)
    return render_template('login.html', form=form)


@app.route('/logout')
def logout():
    app.logger.info('logout user {}'.format(current_user.username))
    logout_user()
    return redirect(url_for('index'))


@app.route('/register', methods=['GET', 'POST'])
def register():
    if current_user.is_authenticated:
        return redirect(url_for('index'))
    form = RegistrationForm()
    if form.validate_on_submit():
        user = User(username=form.username.data, email=form.email.data)
        user.set_password(form.password.data)
        db.session.add(user)
        db.session.commit()
        app.logger.info('registered new user {}'.format(user.username))
        flash('Congratulations, you are now a registered user!')
        return redirect(url_for('login'))
    return render_template('register.html', title='Register', form=form)

```

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

  A) follow
  B) logout
  C) login
  D) game
Answer: C