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, request, redirect, flash, abort
from flask_login import login_required, login_user, logout_user, current_user

from thermos import app, db, login_manager
from thermos.forms import BookmarkForm, LoginForm, SignupForm
from thermos.models import User, Bookmark, Tag



@login_manager.user_loader
def load_user(userid):
	return User.query.get(int(userid))


@app.route('/')
@app.route('/index')
def index():
	return render_template('index.html', new_bookmarks=Bookmark.newest(5))

@app.route('/add', methods=['GET', 'POST'])
@login_required
def add():
	form = BookmarkForm()
	if form.validate_on_submit():		
		url = form.url.data
		description = form.description.data
		tags = form.tags.data
		bm = Bookmark(user=current_user, url=url,  description=description, tags=tags)
		db.session.add(bm)
		db.session.commit()
		flash("Stored '{}'".format(description))
		return redirect(url_for('index'))
	return render_template('bookmark_form.html', form=form, title="Add a bookmark")

@app.route('/edit/<int:bookmark_id>', methods=["GET", "POST"])
@login_required
def edit_bookmark(bookmark_id):
	bookmark = Bookmark.query.get_or_404(bookmark_id)
	if current_user != bookmark.user:
		abort(403)
	form = BookmarkForm(obj=bookmark)
	if form.validate_on_submit():
		form.populate_obj(bookmark)
		db.session.commit()
		flash("Stored '{}'".format(bookmark.description))
		return redirect(url_for('user', username=current_user.username))
	return render_template('bookmark_form.html', form=form, title="Edit bookmark")

@app.route('/delete/<int:bookmark_id>', methods=["GET", "POST"])
@login_required
def delete_bookmark(bookmark_id):
	bookmark = Bookmark.query.get_or_404(bookmark_id)
	if current_user != bookmark.user:
		abort(403)
	if request.method == "POST":
		db.session.delete(bookmark)
		db.session.commit()
		flash("Deleted '{}'".format(bookmark.description))
		return redirect(url_for('user', username=current_user.username))
	else:
		flash("Please confirm deleting the bookmark.")
	return render_template('confirm_delete.html', bookmark=bookmark, nolinks=True)


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


@app.route("/login", methods=["GET", "POST"])
def login():
	form = LoginForm()
	if form.validate_on_submit():
		#login and validate the user...
		user = User.get_by_username(form.username.data)
		if user is not None and user.check_password(form.password.data):
			login_user(user, form.remember_me.data)
			flash("Logged in successfully as {}.".format(user.username))
			return redirect(request.args.get('next') or url_for('user',username=user.username))
		flash("Incorrect username or password")
	return render_template("login.html", form=form)

@app.route("/logout")
def logout():
	logout_user()
	return redirect(url_for('index'))

@app.route("/signup", methods=["GET", "POST"])
def signup():
	form = SignupForm()
	if form.validate_on_submit():
		user = User(email=form.email.data,
					username=form.username.data,
					password=form.password.data)
		db.session.add(user)
		db.session.commit()
		flash('Welcome, {}! Please login.'.format(user.username))
		return redirect(url_for('login'))
	return render_template("signup.html", form = form)

@app.route('/tag/<name>')
def tag(name):
	tag = Tag.query.filter_by(name=name).first_or_404()
	return render_template('tag.html', tag=tag)

@app.errorhandler(404)
def page_not_found(e):
	return render_template('404.html'), 404

@app.errorhandler(500)
def server_error(e):
	return render_template('500.html'), 500

@app.context_processor
def inject_tags():
	return dict(all_tags=Tag.all)



```

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

  A) login
  B) signup
  C) user
  D) server_error
Answer: A