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 flask import render_template, url_for, flash, redirect, request, Blueprint
from flask_login import login_user, current_user, logout_user, login_required
from app import db, bcrypt
from app.models import User, Post
from app.users.forms import (RegistrationForm, LoginForm, UpdateAccountForm,
                                   RequestResetForm, ResetPasswordForm)
from app.users.utils import save_picture, send_reset_email

#instatntiating the blueprint
users = Blueprint('users',__name__)

@users.route("/register", methods=['GET','POST'])
def register():
	if current_user.is_authenticated:
		return redirect(url_for('main.home'))
	form = RegistrationForm()
	if form.validate_on_submit():
		hashed_password = bcrypt.generate_password_hash(form.password.data).decode('utf-8')
		user = User(username=form.username.data, email=form.email.data, password=hashed_password)
		db.session.add(user)
		db.session.commit()

		def send_reset_email(user):
			token = user.get_reset_token()
			user_welcome = Message('User Welcome',
			sender = 'noreply@demo.com', recipients = [user.email] )
			user_welcome.body = '''Login;
			{url_for('main.login.html') }

			If yoy did not make this request, ignore this email and no changes will be made;

			'''
			mail.send(user_welcome)

		flash(f'Account created for {form.username.data}. You can now login', 'success')
		return redirect(url_for('main.login'))
	return render_template('register.html', title = 'Register', form = form)

@users.route("/login", methods=['GET','POST'])
def login():
	if current_user.is_authenticated:
		return redirect(url_for('main.home'))
	form = LoginForm()
	if form.validate_on_submit():
		user = User.query.filter_by(email = form.email.data).first()
		if user and bcrypt.check_password_hash(user.password, form.password.data):
			login_user(user, remember = form.remember.data)
			next_page = request.args.get('next')
			#redirect to the next page if it exists, else render home
			#if there is not a next page, always render home
			return redirect(next_page) if next_page else redirect(url_for('main.home'))
		else:
			flash(f'Login Unsuccesfull. Please check email and password.', 'danger')
	return render_template('login.html', title = 'Login', form = form)


#LOGOUT USER
@users.route("/logout")
def logout():
	logout_user()
	return redirect(url_for('main.home'))

	#ACCOUNT
@users.route("/account", methods=['GET','POST'])
@login_required
def account():
	form = UpdateAccountForm()
	if form.validate_on_submit():
		if form.picture.data:
			picture_file = save_picture(form.picture.data)
			current_user.image_file = picture_file
		current_user.username = form.username.data
		current_user.email = form.email.data
		db.session.commit()
		flash('Your account has been updated', 'success')
		return redirect (url_for('main.account'))
	elif request.method == 'GET':
		form.username.data = current_user.username
		form.email.data = current_user.email
	image_file = url_for('static', filename='profile_pics/' + current_user.image_file)
	return render_template('account.html', title = 'Account', 
		image_file = image_file, form=form)


#USERS TO SEE THEIR POSTS WHEN CLICK THEIR USERNAME
@users.route("/user/<string:username>")
def user_posts(username):
	#PAGINATION
	page = request.args.get('page', 1, type = int)
	user = User.query.filter_by(username = username).first_or_404()
	posts = Post.query.filter_by(user = user)\
		.order_by(Post.date_posted.desc())\
		.paginate(page = page, per_page = 5)
	return render_template('user_posts.html', posts=posts, user = user)


@users.route("/reset_password", methods=['GET','POST'])
def reset_request():
	if current_user.is_authenticated:
		return redirect(url_for('main.home'))
	form = RequestResetForm()
	if form.validate_on_submit():
		user = User.query.filter_by(email=form.email.data).first()
		send_reset_email(user)
		flash('An email has been sent to your email account to help with passeord reset.', 'info')
		return redirect(url_for('main.login'))
	return render_template('reset_request.html', title = 'Request Password', form = form)


@users.route("/reset_password/<token>", methods=['GET','POST'])
def reset_token(token):
	if current_user.is_authenticated:
		return redirect(url_for('main.home'))
	user = User.verify_reset_token(token)
	if user is None:
		flash('That is an invalid or expired token', 'warning')
	return redirect(url_for('users.reset_request'))
	form = ResetPasswordForm()
	if form.validate_on_submit():
		hashed_password = bcrypt.generate_password_hash(form.password.data).decode('utf-8')
		user.password = hashed_password
		db.session.commit() 
		flash(f'Password for {form.username.data} update susscessful. You can now login', 'success')
		return redirect(url_for('main.login'))
	return render_template('reset_token.html', title ='Reset Password', form = form)
```

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

  A) login
  B) account
  C) logout
  D) register
Answer: A